
Every JavaScript number is a 64-bit IEEE 754 floating-point value. There is no separate integer type - which means integers above 2⁵³ lose precision, and decimal arithmetic has well-known rounding quirks.

<ExamplePair>
<ExampleProse>

The classic float trap: `0.1 + 0.2` does not equal `0.3`. Compare floating-point numbers using a tolerance instead of strict equality.

</ExampleProse>
<ExampleCode>

```js
console.log(0.1 + 0.2 === 0.3); // false
console.log(0.1 + 0.2); // 0.30000000000000004

// Safe float comparison
const a = 0.1 + 0.2;
const b = 0.3;
console.log(Math.abs(a - b) < Number.EPSILON); // true
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`Number.MAX_SAFE_INTEGER` is the largest integer that can be represented exactly. Beyond it, incrementing no longer works reliably.

</ExampleProse>
<ExampleCode>

```js
console.log(Number.MAX_SAFE_INTEGER); // 9007199254740991

const n = Number.MAX_SAFE_INTEGER;
console.log(n + 1 === n + 2); // true - precision lost
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`BigInt` handles arbitrarily large integers exactly. Use the `n` suffix for literals. BigInt and Number cannot be mixed in arithmetic - you must convert explicitly.

</ExampleProse>
<ExampleCode>

```js
const big = 9007199254740991n + 1n;
console.log(big); // 9007199254740992n

console.log(10n * 2n); // 20n
// console.log(10n + 1); // TypeError: Cannot mix BigInt and other types
```

</ExampleCode>
</ExamplePair>

<InProduction>
Never store monetary values as `Number`. Representing $19.99 as `19.99` and doing arithmetic on it will produce rounding errors that compound across transactions. Store money as integer cents (`1999`) or use a decimal library like [decimal.js](https://mikemcl.github.io/decimal.js/). Payment systems that let floats drift ship silent bugs that accounting finds months later.
</InProduction>


Every non-BigInt number in JavaScript is a 64-bit float - understanding the precision limits prevents hard-to-debug arithmetic bugs.

Numbers


JavaScript strings are sequences of UTF-16 code units, not characters. For most ASCII text this distinction is invisible, but it surfaces immediately when you handle emoji, accented letters, or any character outside the Basic Multilingual Plane.

<ExamplePair>
<ExampleProse>

Template literals use backticks and `${expression}` interpolation. They support multi-line strings and embed any expression - no concatenation required.

</ExampleProse>
<ExampleCode>

```js
const name = "world";
const greeting = `Hello, ${name}!`;
console.log(greeting); // Hello, world!

const multiline = `Line one
Line two`;
console.log(multiline);
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

An emoji like `"😀"` is encoded as a surrogate pair - two UTF-16 code units. The `.length` property counts code units, not visible characters, so it returns `2` for a single emoji.

</ExampleProse>
<ExampleCode>

```js
const emoji = "😀";
console.log(emoji.length); // 2 - two UTF-16 code units

// Spread iterates code points, not code units
console.log([...emoji].length); // 1 - one code point
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`String.prototype.normalize` converts between Unicode normalization forms (NFC, NFD, NFKC, NFKD). Two strings that look identical can compare unequal if one uses a precomposed character and the other uses a base + combining mark.

</ExampleProse>
<ExampleCode>

```js
const a = "é"; // é - precomposed
const b = "é"; // é - e + combining acute accent

console.log(a === b); // false
console.log(a.normalize() === b.normalize()); // true
```

</ExampleCode>
</ExamplePair>

<InProduction>
Any string you truncate for a UI label, store in a database with a character-limit column, or split on "characters" needs [`Intl.Segmenter`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Segmenter) or a grapheme-cluster library. A naive `.slice(0, 10)` on a user's display name can cut a surrogate pair in half, writing a lone surrogate to the database and producing mojibake on read-back. Family emoji (`👨‍👩‍👧`) can be 8+ code units - `.length` will not tell you what you think.
</InProduction>


JavaScript strings are sequences of UTF-16 code units - template literals, surrogate pairs, and grapheme clusters each have important implications.

Strings


`const` locks a variable's _binding_ - you cannot reassign the name. It does not make the value immutable.

<ExamplePair>
<ExampleProse>

Attempting to reassign a `const` binding throws a `TypeError` at runtime and is caught at parse time by TypeScript and most linters.

</ExampleProse>
<ExampleCode>

```js
const x = 1;
x = 2; // TypeError: Assignment to constant variable.
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

An object bound by `const` is still mutable. Its properties can be changed freely because the binding (the pointer) is fixed, not the data it points to.

</ExampleProse>
<ExampleCode>

```js
const config = { retries: 3 };
config.retries = 5; // fine - mutating the object, not the binding
console.log(config.retries); // 5
```

</ExampleCode>
</ExamplePair>

<ExamplePair>
<ExampleProse>

`Object.freeze` prevents property mutation. Writes fail silently in sloppy mode and throw a `TypeError` in strict mode. Note that `freeze` is shallow - nested objects remain mutable.

</ExampleProse>
<ExampleCode>

```js
"use strict";

const settings = Object.freeze({ timeout: 5000 });
settings.timeout = 1000; // TypeError: Cannot assign to read only property
```

</ExampleCode>
</ExamplePair>

<InProduction>
Teams that read `const` as "immutable" and treat a shared config object as safe to pass around are in for a surprise when two call sites mutate it concurrently. Deep immutability is a library concern - [Immer](https://immerjs.github.io/immer/) for update workflows, `structuredClone` for a quick deep copy - or a TypeScript type concern (`as const`, `Readonly<T>`). The language keyword alone doesn't buy it.
</InProduction>


const locks the binding, not the value - objects and arrays declared with const are still mutable.