How to create static array in JavaScript

Question

I am trying to create a static array. But I found that it can increase the item in the array at run time. How I can achieve static array in JavaScript? Why array is mutable in JavaScript?

var a = [];
//var a = new Array(3);
for (var i = 0; i < 5; i++) {
  //a.push(i);
  a[i] = i;
}

document.write(a);

Answer 1

You can freeze the array with Object.freeze:

"use strict";
var a = Object.freeze([0, 1, 2]);
console.log(a);
try {
  a.push(3);     // Error
} catch (e) {
  console.error(e);
}
try {
  a[0] = "zero"; // Error
} catch (e) {
  console.error(e);
}
console.log(a);

That disallows

• Changing existing properties, either their values or their other features like whether they're extensible, etc.
• Adding or removing properties

See the link for full details. If you just want to keep the size fixed but do want to allow changes to the values of entries, just use Object.seal instead.

Note that whether attempts to change existing properties result in an error (as opposed to silent failure) depends on whether you're in strict mode.

freeze and seal were introduced in ES5 (June 2009), and so should be present in any vaguely up-to-date browser. Obsolete browsers will not have them.