将对象属性添加到JSON对象中

Question

I have a JSON object in this format.

[
    {
        "name": "schoolname",
        "line id": "0",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    },
    {
        "name": "studentname1",
        "line id": "1",
        "class": "A"
    },
    {
        "name": "studentname2",
        "line id": "2",
        "class": "B"
    }
]

What I want to do

From a set of specified headers, get it from the "line id" : "0" and set it to the other items.

For Example: headers = ["time", "minage", "maxage"]

I get these from the "line id" : "0" and give it to others like this.

[
    {
        "name": "schoolname",
        "line id": "0",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    },
    {
        "name": "studentname1",
        "line id": "1",
        "class": "A",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    },
    {
        "name": "studentname2",
        "line id": "2",
        "class": "B",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    }
]

and then remove the element with "line id" : "0" , like this:

[
    {
        "name": "studentname1",
        "line id": "1",
        "class": "A",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    },
    {
        "name": "studentname2",
        "line id": "2",
        "class": "B",
        "time": "4-5",
        "minage": "15",
        "maxage": "35"
    }
]

It is said that the 1st element would be a "line id" : "0" .

What I've tried:

var headers = ["time", "minage", "maxage"]
var data = 
    [
        {
            "name": "schoolname",
            "line id": "0",
            "time": "4-5",
            "minage": "15",
            "maxage": "35"
        },
        {
            "name": "studentname1",
            "line id": "1",
            "class": "A"
        },
        {
            "name": "studentname2",
            "line id": "2",
            "class": "B"
        }
    ];

for(var i = 1; i < data.length; i++)//iterate over the data leaving the 1st line
{
    for(var j = 0; j < headers.length; j++)//add each header to the data lines
     {
        data[i][headers[j]] = data[0][headers[j]];
     }
}
data.splice(0,1);

Everything works fine and as intended. Is there a way to reduce the time-complexity of this and make it more efficient.

This now has a time complexity of O(n * m).

Is there a way around of adding these few objects to all the elements? As the key value pairs to be added for all the entries remain the same.

Answer 1

You can use Object.defineProperties like

 var arr = [{ "name": "schoolname", "line id": "0", "time": "4-5", "minage": "15", "maxage": "35" }, { "name": "studentname1", "line id": "1", "class": "A" }, { "name": "studentname2", "line id": "2", "class": "B" }], headers = ["time", "minage", "maxage"]; function addHeaders(arr, headers) { var header = arr.splice(0, 1)[0], propObj = headers.reduce(function(acc, el) { acc[el] = { value: header[el], writable: true, enumerable: true }; return acc; }, {}); for (var i = 0, len = arr.length; i < len; i++) { Object.defineProperties(arr[i], propObj); } return arr; } document.getElementById('r').innerHTML = 'initial: ' + JSON.stringify(arr,null,2) + '<br/>'; document.getElementById('r').innerHTML += 'result: ' + JSON.stringify(addHeaders(arr, headers),null,2); 

 <pre id="r"></pre> 

Answer 2

Is that data format you have fixed? You should consider doing something more like

school
    -> info (name, etc.)
    -> [classes]
       -> info
       -> [student_ids]
    -> [students]
       -> info (id)

If you can't change your format. You could do something like what you want with Underscore.js#default . Assuming line_id=0 is always data[0] :

var keys = ['minage','maxage','time'];
var temp = _.pick(data.shift(),keys);
data.forEach(function(e, i, a) {
    a[i] = _.default(e,temp);
});

It doesn't really reduce your complexity because you're basically looking an array of size N and update properties count M, meaning you'll have a complexity of O(N*M). If you want something less complex, don't move/copy the data. Reuse it in the current form.

Answer 3

Since you say that you are copying the same values from the 0th element, you can store it in a variable(say new_data ) and then iterate through the data array and just add them there.
This is as complex as iterating through data and inserting key-val pairs.
Something like this -

> new_data = {}
//Getting all the content with header keys in data into new_data
> headers.forEach(function(v){new_data[v] = data[0][v]})

//Storing the new_data
> new_data
Object {time: "4-5", minage: "15", maxage: "35"}

//Adding the new_data into data
> data.forEach(function(d_val){
    for(k_nd in new_data){
        d_val[k_nd] = new_data[k_nd];
    }
  });

//Removing the 0th array element
> data.splice(0, 1)

//Checking it 
> JSON.stringify(data[0])
"{"name":"studentname1","line id":"1","class":"A","time":"4-5","minage":"15","maxage":"35"}"

Answer 4

Using lodash library:

 var headers = ["time", "minage", "maxage"]; var data = [{ "name": "schoolname", "line id": "0", "time": "4-5", "minage": "15", "maxage": "35" }, { "name": "studentname1", "line id": "1", "class": "A" }, { "name": "studentname2", "line id": "2", "class": "B" }]; var temp = _.pick(data[0], headers); data.splice(0, 1); for (var i = 0; i < data.length; i++) { _.merge(data[i], temp); } var result = JSON.stringify(data); $('#result').text(result); 

 <script src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.0/lodash.min.js"></script> <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <div id="result"></div> 

Answer 5

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EDIT: Found a significant performance booster, faster than all other tested solutions so far: extracting one of the loops and applying via: new Function(...) . Essentially an eval -like approximation of Object.defineProperties(...) . Have added this to the performance tests below:

 function addHeadersNewFunc(arr, headers) { //console.time('addHeadersNewFunc'); var header = arr.shift(), funcBody = ['return item;'], headerPropName, setProps; for(var h = headers.length; h--;) { headerPropName = headers[h]; funcBody.unshift('item["' + headerPropName + '"]="' + header[headerPropName] + '";'); //unshift since loop is reversed and we want props in same add order as other implementations, and we've already added our first line } setProps = new Function('item', funcBody.join('')); //warning, this is a form of 'eval()'... for (var i = arr.length; i--;) { setProps(arr[i]); } //console.timeEnd('addHeadersNewFunc'); return arr; } 

---

Some interesting results testing a few different approaches. I've only just whipped up the performance testing code, happy for any recommended improvements. I've also added some additional implementations - a string replacement approach, and a lazy getter.

In general it looks like the original loop outperforms most of the other suggestions; except for the implementation by @Chris Anderson-MSFT using Underscore's defaults when tested in Chrome, which appeared to actually be faster (didn't fare as well in IE, however). Otherwise, the lazies performed consistently well, also. (* EDIT: as per above, implementation using new Function() eventually found to be fastest; for large objects/iterations, significantly).

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Sample output of the below snippet (Chrome 43):

 Items: 2000 Output of functions is all consistent: true Testing... addHeadersOrig x 1000: [Avg] 2.3977ms, [Min] 2.3170ms, [Max] 2.8280ms addHeadersDefineProp x 1000: [Avg] 6.3481ms, [Min] 6.1010ms, [Max] 15.1750ms addHeadersStrReplace x 1000: [Avg] 3.0551ms, [Min] 2.6630ms, [Max] 5.9910ms addHeadersUnderscoreDefaults x 1000: [Avg] 1.4344ms, [Min] 1.1800ms, [Max] 9.5100ms addHeadersLazy x 1000: [Avg] 2.4529ms, [Min] 2.3460ms, [Max] 6.0770ms addHeadersLazyMemo x 1000: [Avg] 2.4837ms, [Min] 2.3760ms, [Max] 3.8420ms addHeadersNewFunc x 1000: [Avg] 0.0959ms, [Min] 0.0430ms, [Max] 0.5070ms 

 (function() { "use strict"; var arr = [{ "name": "schoolname", "line id": "0", "time": "4-5", "minage": "15", "maxage": "35" }, { "name": "studentname1", "line id": "1", "class": "A" }, { "name": "studentname2", "line id": "2", "class": "B" }], headers = ["time", "minage", "maxage"]; //add some more... for (var i = 3, iLen = 2000; i < iLen; i++) { arr.push({ name: "studentname" + i, "line id": String(i), "class": "C" }); } function addHeadersOrig(arr, headers) { //console.time('addHeadersOrig'); for (var i = 1; i < arr.length; i++) //iterate over the data leaving the 1st line { for (var j = 0; j < headers.length; j++) //add each header to the data lines { arr[i][headers[j]] = arr[0][headers[j]]; } } arr.splice(0, 1); //console.timeEnd('addHeadersOrig'); return arr; } function addHeadersDefineProp(arr, headers) { //console.time('addHeadersDefineProp'); var header = arr.splice(0, 1)[0], propObj = headers.reduce(function headerReduce(acc, el) { acc[el] = { value: header[el], writable: true, enumerable: true }; return acc; }, {}); for (var i = 0, len = arr.length; i < len; i++) { Object.defineProperties(arr[i], propObj); } //console.timeEnd('addHeadersDefineProp'); return arr; } function addHeadersStrReplace(arr, headers) { //console.time('addHeadersStrReplace'); var header = arr.shift(), propObj = {}; for (var i = 0; i < headers.length; i++) { propObj[headers[i]] = header[headers[i]]; } //stringify the array, replace each '}' with a ',' followed by the the stringified propObj (minus its opening bracket) which brings its own closing bracket to make up for the one we replaced; then parse back to an object arr = JSON.parse(JSON.stringify(arr).replace(/\\}/g, ',' + JSON.stringify(propObj).slice(1))); //console.timeEnd('addHeadersStrReplace'); return arr; } //only runs using lodash, not underscore function addHeadersLodashMerge(arr, headers) { //console.time('addHeadersLodashMerge'); var temp = _.pick(arr.shift(), headers); for (var i = 0; i < arr.length; i++) { _.merge(arr[i], temp); } //console.timeEnd('addHeadersLodashMerge'); return arr; } //runs under both lodash and underscore - faster in underscore AFAICT function addHeadersUnderscoreDefaults(arr, headers) { //console.time('addHeadersUnderscoreDefaults'); var temp = _.pick(arr.shift(), headers); arr.forEach(function(e, i, a) { a[i] = _.defaults(e, temp); }); //console.timeEnd('addHeadersUnderscoreDefaults'); return arr; } function addHeadersNewFunc(arr, headers) { //console.time('addHeadersNewFunc'); var header = arr.shift(), funcBody = ['return item;'], headerPropName, setProps; for(var h = headers.length; h--;) { headerPropName = headers[h]; funcBody.unshift('item["' + headerPropName + '"]="' + header[headerPropName] + '";'); //unshift since loop is reversed and we want props in same add order as other implementations, and we've already added our first line } setProps = new Function('item', funcBody.join('')); //warning, this is a form of 'eval()'... for (var i = arr.length; i--;) { setProps(arr[i]); } //console.timeEnd('addHeadersNewFunc'); return arr; } function addHeadersLazy(arr, headers) { //console.time('addHeadersLazy'); var lazy = new Lazy(arr, headers), result = []; for (var i = 1; i < arr.length; i++) { result.push(lazy.get(i)); } //console.timeEnd('addHeadersLazy'); return result; } function addHeadersLazyMemo(arr, headers) { //console.time('addHeadersLazyMemo'); var lazy = new Lazy(arr, headers, true), result = []; for (var i = 1; i < arr.length; i++) { result.push(lazy.get(i)); } //console.timeEnd('addHeadersLazyMemo'); return result; } function Lazy(arr, headers, useMemo) { var headerValSrc = arr[0], headerLen = headers.length, memo = []; function _get(index) { for (var j = 0; j < headerLen; j++) { arr[index][headers[j]] = headerValSrc[headers[j]]; } return arr[index]; } function _getMemo(index) { if (memo[index]) { return memo[index]; } for (var j = 0; j < headerLen; j++) { arr[index][headers[j]] = headerValSrc[headers[j]]; } return (memo[index] = arr[index]); } return { get: (useMemo ? _getMemo : _get) }; } function clone(data) { return JSON.parse(JSON.stringify(data)); } function perfTest(name, testFunc) { name = name ? name : "Test"; var iterations = 1000, argsSliced = Array.prototype.slice.call(arguments, 2), args = [], t0 = 0, t1, t2, t3, tmin = 1000000, tmax = 0, output; setTimeout(function delayAllowingDocWrite() { for (var i = 0; i < iterations; i++) { args = clone(argsSliced); t1 = performance.now(); testFunc.apply(this, args); t2 = performance.now(); t3 = t2 - t1; tmin = t3 < tmin ? t3 : tmin; tmax = t3 > tmax ? t3 : tmax; t0 += t3; } output = name + " x " + iterations + ": [Avg] " + (t0 / iterations).toFixed(4) + "ms, [Min] " + tmin.toFixed(4) + "ms, [Max] " + tmax.toFixed(4) + "ms"; console.log(output); document.body.innerHTML += (output + "<br />"); }, 10); return testFunc.apply(this, clone(argsSliced)); //return output of function immed, once, for comparing results } document.body.innerHTML += "Items: " + arr.length + "<br />"; console.log("Items: ", arr.length); //* var resultOrig = perfTest("addHeadersOrig", addHeadersOrig, arr, headers), resultDefineProp = perfTest("addHeadersDefineProp", addHeadersDefineProp, arr, headers), resultStrReplace = perfTest("addHeadersStrReplace", addHeadersStrReplace, arr, headers), //resultLodashMerge = perfTest("addHeadersLodashMerge", addHeadersLodashMerge, arr, headers), //re-enable if using lodash.min.js resultUnderscoreDefaults = perfTest("addHeadersUnderscoreDefaults", addHeadersUnderscoreDefaults, arr, headers), resultLazy = perfTest("addHeadersLazy", addHeadersLazy, arr, headers), resultLazyMemo = perfTest("addHeadersLazyMemo", addHeadersLazyMemo, arr, headers), resultNewFunc = perfTest("addHeadersNewFunc", addHeadersNewFunc, arr, headers); //*/ var resultOrigStr = JSON.stringify(resultOrig), outputIsConsistent = "Output of functions is all consistent: " + ( resultOrigStr === JSON.stringify(resultDefineProp) && resultOrigStr === JSON.stringify(resultStrReplace) && //resultOrigStr === JSON.stringify(resultLodashMerge) && resultOrigStr === JSON.stringify(resultUnderscoreDefaults) && resultOrigStr === JSON.stringify(resultLazy) && resultOrigStr === JSON.stringify(resultLazyMemo) && resultOrigStr === JSON.stringify(resultNewFunc) ); document.body.innerHTML += outputIsConsistent + "<br /><em>Testing...</em><br /><br />"; console.log(outputIsConsistent); if (!window.performance || !window.performance.now) { document.body.innerHTML += "Your browser does not seem to support performance.now()..."; } /* var arr1 = clone(arr), arr2 = clone(arr), arr3 = clone(arr), arr4 = clone(arr), arr5 = clone(arr), arr6 = clone(arr); var resultOrig = addHeadersOrig(arr1, headers), resultDefineProp = addHeadersDefineProp(arr2, headers), resultStrReplace = addHeadersStrReplace(arr3, headers), resultLodash = addHeadersLodash(arr4, headers), resultLazy = addHeadersLazy(arr5, headers), resultLazyMemo = addHeadersLazyMemo(arr6, headers); console.log(resultOrig); console.log(resultDefineProp); console.log(resultStrReplace); console.log(resultLodash); console.log(resultLazy); console.log(resultLazyMemo); //*/ })(); 

 body { font-size: 0.8em; font-family: "Arial", sans-serif; } 

 <!--script src="//cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.0/lodash.min.js"></script--> <script src="//cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script> <p>Use a browser that supports performance.now().</p> 

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For easier playing around with: Plnkr