JavaScript — Assignment

= means "store this"

Assignment puts a value into a variable:

let score = 0;          // create + initialize
score = 50;             // overwrite
score = score + 10;     // read current, add 10, store back

Read it right-to-left: "compute the right side, put the result into the left name."

Compound assignment — update in place

Every math operator has an "apply to self" shortcut:

score += 10    // same as: score = score + 10
score -= 5     // subtract from self
score *= 2     // double it
score /= 3     // halve-ish it
count %= 7     // remainder-into-self

Statement	Long form
x += y	x = x + y
x -= y	x = x - y
x *= y	x = x * y
x /= y	x = x / y

Real-world shape:

cartTotal += item.price;      // accumulate purchases
attempts -= 1;                // burn a try
progress += 100 / steps;      // percent complete

Chained assignment (works, rarely wise)

let a = b = c = 0;   // all zero — but b,c become implicit globals in sloppy mode

Modern style declares each separately:

let a = 0;
let b = 0;

The three-stripe confusion

These symbols look related; they're entirely different:

Symbol	Meaning	Type
=	assign	statement action
==	loose compare (converts!)	question → boolean
===	strict compare	question → boolean
=>	arrow function	syntax

if (x = 5) { … }    // BUG! assigned 5 (truthy) — always runs!
if (x === 5) { … }  // correct comparison

That single-character typo is a classic bug — good editors warn about assignment-in-condition for exactly this reason.

Destructuring — modern unpacking

Assignment can pull apart arrays and objects:

const [first, second] = ["Ada", "Grace"];
console.log(first);            // "Ada"

const { name, age } = { name: "Ada", age: 36 };
console.log(name, age);        // Ada 36

// swapping without a temp variable:
[a, b] = [b, a];

You'll meet this constantly in modern codebases (and React).

Mini Practice

1. Build a counter using only +=, -=, *= in sequence
2. Trigger the if (x = 5) bug on purpose; then fix with ===
3. Destructure two properties from an object into variables
4. Swap two variables via destructuring
5. Predict-then-run: does x += "5" on number 1 give 6 or "15"? Explain why

Next: data types → (already written)