Constants and Object.freeze in JavaScript
Node.js already supports const and most browsers eventaully will, but that only creates immutable variables not objects. For example:
const obj = { property1: 'value1' }; // some implementations will error out, some will just not assign to it. try { obj = { property2: 'value2' }; } catch (e) { console.log(obj); } console.log(obj); // { property1: 'value1' } obj.property1 = 'value2'; console.log(obj); // { property1: 'value2' }
This is where Object.freeze comes in, it prevents the entire object from being modified:
const obj = Object.freeze({ property1: 'value1' }); // some implementations will error out, some will just not assign to it. try { obj.property1 = 'value2'; } catch (e) {} console.log(obj); // { property1: 'value1' }
This is useful for the same reason that const is useful, it makes code more predictable by ensuring that an object’s properties and values can always be relied on once defined. Though it does have a weakness with arrays:
const arr = Object.freeze([{ property1: 'value1' }, { property2: 'value2' }]); // some implementations will error out, some will just not assign to it. try { arr[0] = { property2: 'value2' }; } catch (e) {} arr[0].property1 = 'value2'; console.log(arr);
So it is important to remember to use Object.freeze on each array element:
const arr2 = [{ property1: 'value1' }, { property2: 'value2' }].map(Object.freeze); // some implementations will error out, some will just not assign to it. try { arr2[0].property1 = 'value2'; } catch (e) {} console.log(arr2); // [ { property1: 'value1' }, { property2: 'value2' } ]
Although, this does not seem like something that has good support, it actually works in all modern browsers. Though since Object.freeze is most useful with const and const is still not supported in some environments, you will need another way of generating constants. Finally, frozen objects run just as fast as regular objects. Frozen objects only ran slower in the past due to browser bugs. For example: https://bugs.chromium.org/p/chromium/issues/detail?id=115960.
Github Location: https://github.com/Jacob-Friesen/obscurejs/blob/master/2016/objectFreeze.js
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