Why does Array.prototype.push return the new length instead of something more useful?

Question

Ever since its introduction in ECMA-262, 3rd Edition, the Array.prototype.push method\'s return value is a Number:

Answer 1

I cannot explain why they chose to return the new length, but in response to your suggestions:

• Returning the newly appended item:

Given that JavaScript uses C-style assignment which emits the assigned value (as opposed to Basic-style assignment which does not) you can still have that behavior:

var addedItem;
myArray.push( addedItem = someExpression() );

(though I recognise this does mean you can't have it as part of an r-value in a declaration+assignment combination)

• Returning the mutated array itself:

That would be in the style of "fluent" APIs which gained popularity significantly after ECMAScript 3 was completed and it would not be keeping in the style of other library features in ECMAScript, and again, it isn't that much extra legwork to enable the scenarios you're after by creating your own push method:

Array.prototype.push2 = function(x) {
    this.push(x);
    return this;
};

myArray.push2( foo ).push2( bar ).push2( baz );

or:

Array.prototype.push3 = function(x) {
    this.push(x);
    return x;
};

var foo = myArray.push3( computeFoo() );