I have json based data structure with objects containing nested objects. In order to access a particular data element I have been chaining references to object properties together. For example:
var a = b.c.d;
If b or bc is undefined, this will fail with an error. However, I want to get a value if it exists otherwise just undefined. What is the best way to do this without having to check that every value in the chain exists?
I would like to keep this method as general as possible so I don't have to add huge numbers of helper methods like:
var a = b.getD();
or
var a = helpers.getDFromB(b);
I also want to try to avoid try/catch constructs as this isn't an error so using try/catch seems misplaced. Is that reasonable?
Any ideas?
Standard approach:
var a = b && b.c && b.c.d && b.c.d.e;
is quite fast but not too elegant (especially with longer property names).
Using functions to traverse JavaScipt object properties is neither efficient nor elegant.
Try this instead:
try { var a = b.c.d.e; } catch(e){}
in case you are certain that a
was not previously used or
try { var a = b.c.d.e; } catch(e){ a = undefined; }
in case you may have assigned it before.
This is probably even faster that the first option.
ECMAScript2020 , and in Node v14, has the optional chaining operator (I've seen it also called safe navigation operator), which would allow your example to be written as:
var a = b?.c?.d;
From the MDN docs :
The optional chaining operator (?.) permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid. The ?. operator functions similarly to the . chaining operator, except that instead of causing an error if a reference is nullish (null or undefined), the expression short-circuits with a return value of undefined. When used with function calls, it returns undefined if the given function does not exist.
You can create a general method that access an element based on an array of property names that is interpreted as a path through the properties:
function getValue(data, path) {
var i, len = path.length;
for (i = 0; typeof data === 'object' && i < len; ++i) {
data = data[path[i]];
}
return data;
}
Then you could call it with:
var a = getValue(b, ["c", "d"]);
This is an old question and now with es6 features, this problem can be solved more easily.
const idx = (p, o) => p.reduce((xs, x) => (xs && xs[x]) ? xs[x] : null, o);
Thanks to @sharifsbeat for this solution .
ES6 has optional chaining which can be used as follows:
const object = { foo: {bar: 'baz'} }; // not found, undefined console.log(object?.foo?.['nested']?.missing?.prop) // not found, object as default value console.log(object?.foo?.['nested']?.missing?.prop || {}) // found, "baz" console.log(object?.foo?.bar)
This approach requires the variable "object" to be defined and to be an object.
Alternatively, you could define your own utility, here's an example which implements recursion:
const traverseObject = (object, propertyName, defaultValue) => { if (Array.isArray(propertyName)) { return propertyName.reduce((o, p) => traverseObject(o, p, defaultValue), object); } const objectSafe = object || {}; return objectSafe[propertyName] || defaultValue; }; // not found, undefined console.log(traverseObject({}, 'foo')); // not found, object as default value console.log(traverseObject(null, ['foo', 'bar'], {})); // found "baz" console.log(traverseObject({foo: {bar:'baz'}}, ['foo','bar']));
probably it's may be simple:
let a = { a1: 11, b1: 12, c1: { d1: 13, e1: { g1: 14 }}}
console.log((a || {}).a2); => undefined
console.log(((a || {}).c1 || {}).d1) => 13
and so on.
The answers here are good bare-metal solutions. However, if you just want to use a package that is tried and true, I recommend using lodash.
With ES6 you can run the following
import _ from 'lodash'
var myDeepObject = {...}
value = _.get(myDeepObject, 'maybe.these.path.exist', 'Default value if not exists')
const getValue = (obj, property, defaultValue) => (
property.split('.').reduce((item, key) => {
if (item && typeof item === 'object' && key in item) {
return item[key];
}
return defaultValue;
}, obj)
)
const object = { 'a': { 'b': { 'c': 3 } } };
getValue(object, 'a.b.c'); // 3
getValue(object, 'a.b.x'); // undefined
getValue(object, 'a.b.x', 'default'); // 'default'
getValue(object, 'a.x.c'); // undefined
I will just paste the function that I use in almost all project as utility for this type of situation.
public static is(fn: Function, dv: any) {
try {
if (fn()) {
return fn()
} else {
return dv
}
} catch (e) {
return dv
}
}
So first argument is callback and second is the default value if it fails to extract the data due to some error.
I call it at all places as follows:
var a = is(()=> a.b.c, null);
// The code for the regex isn't great,
// but it suffices for most use cases.
/**
* Gets the value at `path` of `object`.
* If the resolved value is `undefined`,
* or the property does not exist (set param has: true),
* the `defaultValue` is returned in its place.
*
* @param {Object} object The object to query.
* @param {Array|string} path The path of the property to get.
* @param {*} [def] The value returned for `undefined` resolved values.
* @param {boolean} [has] Return property instead of default value if key exists.
* @returns {*} Returns the resolved value.
* @example
*
* var object = { 'a': [{ 'b': { 'c': 3 } }], b: {'c-[d.e]': 1}, c: { d: undefined, e: 0 } };
*
* dotGet(object, 'a[0].b.c');
* // => 3
*
* dotGet(object, ['a', '0', 'b', 'c']);
* // => 3
*
* dotGet(object, ['b', 'c-[d.e]']);
* // => 1
*
* dotGet(object, 'c.d', 'default value');
* // => 'default value'
*
* dotGet(object, 'c.d', 'default value', true);
* // => undefined
*
* dotGet(object, 'c.d.e', 'default value');
* // => 'default value'
*
* dotGet(object, 'c.d.e', 'default value', true);
* // => 'default value'
*
* dotGet(object, 'c.e') || 5; // non-true default value
* // => 5
*
*/
var dotGet = function (obj, path, def, has) {
return (typeof path === 'string' ? path.split(/[\.\[\]\'\"]/) : path)
.filter(function (p) { return 0 === p ? true : p; })
.reduce(function (o, p) {
return typeof o === 'object' ? ((
has ? o.hasOwnProperty(p) : o[p] !== undefined
) ? o[p] : def) : def;
}, obj);
}
If you would like to have a dynamic access with irregular number of properties at hand, in ES6 you might easily do as follows;
function getNestedValue(o,...a){ var val = o; for (var prop of a) val = typeof val === "object" && val !== null && val[prop] !== void 0 ? val[prop] : undefined; return val; } var obj = {a:{foo:{bar:null}}}; console.log(getNestedValue(obj,"a","foo","bar")); console.log(getNestedValue(obj,"a","hop","baz"));
Gets the value at path
of object
. If the resolved value is undefined
, the defaultValue
is returned in its place.
In ES6 we can get nested property from an
Object
like below code snippet .
const myObject = { a: { b: { c: { d: 'test' } } }, c: { d: 'Test 2' } }, isObject = obj => obj && typeof obj === 'object', hasKey = (obj, key) => key in obj; function nestedObj(obj, property, callback) { return property.split('.').reduce((item, key) => { if (isObject(item) && hasKey(item, key)) { return item[key]; } return typeof callback != undefined ? callback : undefined; }, obj); } console.log(nestedObj(myObject, 'abcd')); //return test console.log(nestedObj(myObject, 'abcde')); //return undefined console.log(nestedObj(myObject, 'c.d')); //return Test 2 console.log(nestedObj(myObject, 'd.d', false)); //return false console.log(nestedObj(myObject, 'a.b')); //return {"c": {"d": "test"}}
An old question, and now days we have Typescript projects so often that this question seems irrelevant, but I got here searching for the same thing, so I made a simple function to do it. Your thoughts about not using try/catch is too strict for my taste, after all the seek for undefined.x
will cause an error anyway. So with all that, this is my method.
function getSafe (obj, valuePath) {
try { return eval("obj." + valuePath); }
catch (err) { return null; }
}
To use this we have to pass the object. I tried to avoid that, but there was not other way to get scope into it from another function (there is a whole bunch of questions about this in here). And a small test set to see what we get:
let outsideObject = {
html: {
pageOne: {
pageTitle: 'Lorem Ipsum!'
}
}
};
function testme() {
let insideObject = { a: { b: 22 } };
return {
b: getSafe(insideObject, "a.b"), // gives: 22
e: getSafe(insideObject, "a.b.c.d.e"), // gives: null
pageTitle: getSafe(outsideObject, "html.pageOne.pageTitle"), // gives: Lorem Ipsum!
notThere: getSafe(outsideObject, "html.pageOne.pageTitle.style") // gives: undefined
}
}
testme();
UPDATE: Regarding the use of eval
I think that eval is a tool to use carefully and not the devil itself. In this method, the user does not interfere with eval since it is the developer that is looking for a property by its name.
If you care about syntax, here's a cleaner version of Hosar's answer:
function safeAccess(path, object) {
if (object) {
return path.reduce(
(accumulator, currentValue) => (accumulator && accumulator[currentValue] ? accumulator[currentValue] : null),
object,
);
} else {
return null;
}
}
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