如何检查 item 是否是循环中满足特定条件的最后一个元素

Question

let's say I have arbitrary data which looks like this

data= [{foo: bar},{foo: false},{foo:bar},{foo: false}];

and I am looping through that data using JQUERY. How do I check if an iteration is the last to fulfil a certain condition, for example;

$.each(data, function(key, value){


    if (value.foo===bar) {
        //do something
        // if this is the last which has a foo which is === bar then do something
    }

});

Answer 1

You can simulate a map + filter using reduce to do this:

 var data = [{foo : 'bar'}, {foo : false}, {foo : 'bar'}, {foo : false}]; var elems = data.reduce(function(filtered, obj, index) { if (obj.foo === 'bar') { filtered.push(index); } return filtered; }, []); var last_index = elems[elems.length - 1]; data.forEach((value, index) => { if (value.foo === 'bar') { //do something //if this is the last which has a foo which is === bar then do something if (index === last_index) { // do the thing console.log('last: ', value, index); } } });

Or, you can work with data backwards, and use a boolean to detect whether you have seen the last element or not:

 var data = [{foo : 'bar'}, {foo : false}, {foo : 'bar'}, {foo : false}]; var last = true; $.each(data.reverse(), function(key, value) { if (value.foo === 'bar') { //do something //if this is the last which has a foo which is === bar then do something if (last) { // do the thing console.log('last: ', value, data.length - key - 1); last = false; } } });

 <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

Answer 2

Use a regular for loop and iterate in reverse order over your collection so the first match would be the last match in original order

 let data = [{ foo: 'bar' }, { foo: false }, { foo: 'bar' }, { foo: false }]; for (let i = data.length - 1, found = false; i >= 0; i--) { if (!found && data[i].foo === 'bar') { console.log('"last" match at ', i); found = true; } // ... }

Answer 3

you can store the last foo which is equal to bar in a variable and do operation on it outside the loop.

See below sample code

var lastFooEqBar= null;
$.each(data.reverse(), function(key, value) {
    if (value.foo === bar) {
        //do something
        //store in variable
        lastFooEqBar = value;
    }
});
if(null!=lastFooEqBar) {
  //do operation on last foo equals to bar
}