JAVASCRIPT

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Last update:   17-01-2022

If

The if (...) statement evaluates the expression in its parantheses.
 
/**
 * The if (…) statement evaluates the expression in its parentheses ...
 * and converts the result to a boolean.
 * 
 * If the result is true executes the block of code.
 * 
 */

let age = 20;

if (age > 18) {
    console.log("Allowed");
}

Ternary

With conditional operator we can assign a result in a simpler way.
 
/**
 * The ternary operator has three operands.
 * It's the only operator witch has more than two operands.
 */

let age = 20
let allowed = age > 18 ? 'Yes' : 'No';

console.log(allowed); // Yes

Or

In Javascript this operator has some powerful uses.
 
/**
 * In classical programming, the logical OR is for manipulating ...
 * booleans only.
 * 
 * In Javascript the operator is a bit trickier and more powerful.
 * If an operand is not boolean, it's converted to a boolean.
 * 
 * If all operands have been evaluated false, returns the last operand.
 */

console.log(true || false); // true
console.log(false || false); // false

console.log(1 || 0); // 1, like (true || false)
console.log(null || false || '0'); // 0

let firstName = '';
let lastName = '';
let nickName = null;

console.log(firstName || lastName || nickName || 'Anonymous');
    // Anonymous


/**
 * AND
 * Just as with OR, any value is allowed as an operand.
 * The differents is that AND returns first falsy value 
 * (or the last if none found)
 * 
 * The OR return the first truly one.
 */

console.log( 1 && 0); // 0
console.log( 0 && 'no matter what'); // 0

Nullish

This is a recent addition to the language.
 
/**
 * The two question marks operator ?? ...
 * returns the first argument if it's not null/undefined.
 */

let a;
let b = 2;

// classical
let c = (a !== null && a !== undefined) ? a : b; // 2
console.assert( c === b); // pass

// nullish coalescing
let d = a ?? b; // 2
console.assert( d === b); // pass

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