JavaScript basics

Variables and values

Use const by default, let when a binding must be reassigned, and avoid new uses of var:

const course = 'frontend';
let lesson = 2;
lesson += 1;
var legacyName = 'old style';

JavaScript has primitive values such as strings, numbers, booleans, null, undefined, bigint, and symbols. Objects and arrays are reference values:

const name = 'Ada';
const score = 42;
const active = true;
const student = { name, score };
const scores = [42, 98];

console.log(typeof name, typeof student, Array.isArray(scores));

typeof null is a historical language quirk and returns "object"; use value === null when checking for null.

Expressions and control flow

Template literals interpolate values:

const message = `Student: ${student.name}, score: ${student.score}`;

Use strict equality unless coercion is intentional:

0 == false   // true: coercion
0 === false  // false: different types

Conditions use truthiness. Be careful with 0, '', false, null, and undefined when an actual value may be missing.

if (scores.length > 0) {
  for (const score of scores) {
    console.log(score);
  }
} else {
  console.log('No scores');
}

Errors can be handled at a boundary:

try {
  JSON.parse('{bad json}');
} catch (error) {
  console.error('Could not read data', error);
}

Scope

let and const are block-scoped; a binding created inside { ... } is not available outside it. var is function-scoped, which is one reason old loop and callback code can behave unexpectedly. Closures and per-iteration bindings are covered in Functions and closures.

Exercise

Write a validateName(value) function that returns true only for a non-empty string after trimming. Test it with values from a form without using a framework.