Different result when calling Array.prototype.slice.apply(arguments)

Question

I am trying to understand why test3 and test1 produce the same result, which is an array [1,2] but test2 produces an empty array. Isn't the Function property arguments returning an array like object? I expect all three test results to produce an array of [1,2].

/*eslint-disable no-console */
var testFunc = function(){
    console.log(arguments);
    console.log("test3: ", Array.prototype.slice.apply(arguments));
};

testFunc(1,2);
console.log("test1: ", Array.prototype.slice.apply([1,2]));
console.log("test2: ", Array.prototype.slice.apply({[0]:1, [1]:2}));

T.J. Crowder · Accepted Answer · 2020-05-18 14:15:29Z

test2 doesn't produce the result you expect because you're calling it on a non-array-like object: it's missing length. test1 and test3 produce the result you expect because you're calling it on array-like objects. Since test2 is missing length, when slice reads length, it gets undefined, which gets converted to NaN, so 0 gets used as the length of the source array. (If you're curious, in the spec it's slice -> LengthOfArrayLike(obj) -> ToLength(obj.length) -> ToInteger(undefined) -> ToNumber(undefined); ToNumber returns NaN, which ToInteger converts to 0.)

If we add length to test2, it also produces what you expect:

/*eslint-disable no-console */
var testFunc = function(){
    console.log(arguments);
    console.log("test3: ", Array.prototype.slice.apply(arguments));
};

testFunc(1,2);
console.log("test1: ", Array.prototype.slice.apply([1,2]));
console.log("test2: ", Array.prototype.slice.apply({0:1, 1:2, length:2}));

.as-console-wrapper {
    max-height: 100% !important;
}

Side note: No need for computed property notation when building the array-like object for test2, numeric literals are valid property names in object literals (I've removed the computed notation in the snippet above).

Bergi · Accepted Answer · 2020-05-18 14:09:42Z

1

For an object to be array-like, it needs to have a .length property:

console.log("test2: ", Array.prototype.slice.apply({0:1, 1:2, length: 2}));

answered May 18, 2020 at 14:09

Bergi

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