JavaScript Const

A name you do not rebind

const declares a name and assigns it once. You cannot write = to that name again. The binding is fixed. That is the rule.

const days = 7;
console.log(days);
// days = 8; // TypeError: Assignment to constant variable

Use const for values that are facts in this program: a tax rate, a maximum, a heading string you will not replace.

You must assign on the same line

let can wait. const cannot. A declaration without a value is a syntax error.

const title = "StudyGrid";
console.log(title);
// const empty; // SyntaxError: Missing initializer

If you do not know the value yet, use let. Switch to const when the value is known and will not be replaced.

Objects can still change

const locks the binding, not the insides of an object. You cannot point the name at a different object. You can change a property on the object that is already there.

const person = { name: "Ada" };
person.name = "Grace";
console.log(person.name);
// person = { name: "Alan" }; // TypeError: Assignment to constant variable

person.name = "Grace" edits the object. person = ... tries to rebind the name. The first is allowed. The second is not.

Arrays can still change

The same split applies to arrays. You can push, pop, and assign an index. You cannot replace the whole array with a new one through that name.

const colors = ["red", "green"];
colors.push("blue");
console.log(colors);
// colors = []; // TypeError: Assignment to constant variable

const is still the right default for arrays and objects you build once and then fill. You are promising not to swap the box, not that the contents are frozen.

const vs let

Both are block-scoped. Both refuse a second declaration in the same block. The difference is rebinding.

Start with const

A useful habit: declare with const. If the next line needs to assign again, change that one name to let. Most names in a small script never need to be rebound.

What comes next

You can now name a value and decide whether that name may move. Next: operators — the symbols that compute and compare.