Is it an antipattern to set an array length in JavaScript?

Question

Is it bad to use code like:

var a = [1,2,3,4];
a.length = 2; // 3 and 4 are removed

Does it have decent browser support? Do the removed val

Answer 1

Does it have decent browser support?

Yes. This has been present since the very first edition of ECMAScript:

Specifically, whenever a property is added whose name is an array index, the length property is changed, if necessary, to be one more than the numeric value of that array index; and whenever the length property is changed, every property whose name is an array index whose value is not smaller than the new length is automatically deleted.

Standard ECMA-262, 15.4

In ECMAScript 5, this was moved to 15.4.5.2.

Attempting to set the length property of an Array object to a value that is numerically less than or equal to the largest numeric property name of an existing array indexed non-deletable property of the array will result in the length being set to a numeric value that is one greater than that largest numeric property name.

Standard ECMA-262, 15.4.5.2

All browsers support this behavior.

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Do the removed values get garbage collected properly?

Yes. (See quotes above, and 15.4.5.1.)

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Is it an antipattern to set array length in Javascript?

No, not really. While arrays are "exotic objects" in ES6-speak, the real crazily unique thing about arrays are their indexing, not setting the length property. You could replicate the same behavior with a property setter.

It's true that property setters with non-subtle effects are somewhat unusual in JS, as their side-effects can be unobvious. But since .length has been there in Javascript from Day 0, it should be fairly widely understood.

If you want an alternative, use .splice():

// a is the array and n is the eventual length

a.length = n;

a.splice(n, a.length - n); // equivalent
a.splice(n, a.length);     // also equivalent

If I were avoid setting .length for some reason, it would be because it mutates the array, and I prefer immutable programming where reasonable. The immutable alternative is .slice().

a.slice(0, n);  // returns a new array with the first n elements