JavaScript: variables, types and equality

let, const and var with their scope, primitive types and typeof, implicit conversions, == versus === and the falsy values.

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3 topics
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let and const: what is it?
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The two modern variable declarations, limited to the block where they are written; the first allows reassigning, the second forbids it.
const freezes the link between the name and the value, not the content: const t = []; t.push(1) works, t = [] causes TypeError: Assignment to constant variable.
What does this program print?

```javascript
console.log(typeof (1 + '2'), typeof ('3' * '4'), typeof null);
`
- string number object
- number number null
- string string object
- string number null
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string number object
1 + '2' concatenates and gives the string '12'. '3' * '4' converts to numbers and gives 12. typeof null is 'object', a historical error kept so as not to break the web.
JavaScript's primitive types are string, number, boolean, undefined, null, symbol and …; everything else (arrays, functions, objects) is of type …, a function being a callable object, even though typeof answers 'function' for it.
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JavaScript's primitive types are string, number, boolean, undefined, null, symbol and bigint; everything else (arrays, functions, objects) is of type object, a function being a callable object, even though typeof answers 'function' for it.
A primitive value cannot be changed: a method such as toUpperCase() returns a new string. An object, however, is changed in place.
var: what is it?
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The old variable declaration, limited to the function and not the block, which exists (holding undefined) before its declaration line.
A var written inside an if stays visible after it, which hides bugs. With let, reading q before the line let q = 1; raises ReferenceError: Cannot access 'q' before initialization.
What happens when this program runs?

```javascript
const prix = 10;
prix = 12;
console.log(prix);
`
- It prints 12, since const only forbids changing the type
- It stops at line 2 with TypeError: Assignment to constant variable.
- It prints 10, line 2 being ignored without an error
- It does not start: SyntaxError as soon as the file is read
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It stops at line 2 with TypeError: Assignment to constant variable.
Reassigning a constant is a runtime error, not a syntax error: line 1 runs, line 2 throws. For a value that must change, declare it with let.

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