Variables

Naming a value

A variable is a name pointing at a value. You make one by declaring it, with const or let:

const name = 'Ada';
let age = 36;

age = age + 1;

console.log(name, age);

Ada 37

Read = as "refers to", not "equals". The name on the left starts pointing at whatever the right side produced.

const by default

Reach for const first and switch to let only when you actually need to reassign. A reader then knows at a glance which names change, which is most of the value.

const taxRate = 0.19;
let runningTotal = 0;

runningTotal = runningTotal + 100;
runningTotal = runningTotal * (1 + taxRate);

console.log(runningTotal.toFixed(2));

119.00

Declare before you use

Using a name before its declaration is an error, and a helpfully specific one:

try {
  console.log(score);
  let score = 10;
} catch (error) {
  console.log(error.name + ': ' + error.message);
}

ReferenceError: Cannot access 'score' before initialization

The name exists from the top of its block but cannot be touched until the declaration runs. That gap has a name, the temporal dead zone, and it exists to turn a silent undefined into a loud error.

One name, one job

A variable can hold any type, and reusing one name for unrelated things makes code hard to follow. Declare a new name instead; they are free.

const priceText = '42';
const price = Number(priceText);

console.log(typeof priceText, typeof price);

string number

Naming

Names are read far more often than they are written. daysUntilRenewal costs nothing over d and answers a question the reader would otherwise have to work out.

• camelCase for variables and functions.
• Say what it holds, not what type it is: customer, not customerObject.
• Booleans read well with is, has or should: isReady, hasAccess.
• Short names are fine in short scopes, such as the index in a loop.