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parameters.js
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parameters.js
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///////////////////////////
function foo(x, y=0) {
return [x, y];
}
foo(1, 2); // [1, 2]
foo(1); // [1, 0]
foo(); // [undefined, 0]
//////////////////////////
function bar({x, y = 5}) {
console.log(x, y);
}
bar({}) // undefined, 5
bar({x: 1}) // 1, 5
bar({x: 1, y: 2}) // 1, 2
bar() // TypeError: Cannot read property 'x' of undefined
//////////////////////////
let x = 99;
function foo(p = x + 1) {
console.log(p);
}
foo() // 100
x = 100;
foo() // 101
////////////////////////
(function (a) {}).length // 1
(function (a = 5) {}).length // 0
(function (a, b, c = 5) {}).length // 2
(function(...args) {}).length // 0
(function (a = 0, b, c) {}).length // 0
(function (a, b = 1, c) {}).length // 1
//////////////////////////
var x = 1;
function f(x, y = x) {
console.log(y);
}
f(2) // 2
//////////////////////////////////
const x = 'outer';
function foo(a = x) {
const x = 'inner';
console.log(a); // outer
}
////////////////////////////////
function format(pattern, ...params) {
return {pattern, params};
}
format(1, 2, 3);
// { pattern: 1, params: [ 2, 3 ] }
format();
// { pattern: undefined, params: [] }
//////////////////////////////////
function sortNumbers() {
return Array.prototype.slice.call(arguments).sort();
}
const sortNumbers = (...numbers) => numbers.sort()
////////////////////////////////////
Math.max(-1, 5, 11, 3)
Math.max(...[-1, 5, 11, 3])
Math.max(-1, ...[-1, 5, 11], 3)
//11
console.log([1, ...[2,3], 4])
//[1, 2, 3, 4]
////////////////////////////////
function f(x, y, z) {
// ...
}
//es5
var args = [0, 1, 2];
f.apply(null, args);
//es6
var args = [0, 1, 2];
f(...args);
//////////////////////////////////
var arr1 = [0, 1, 2];
var arr2 = [3, 4, 5];
//es5
Array.prototype.push.apply(arr1, arr2);
//es6
arr1.push(...arr2);
///////////////////////////////////
[...'hello']
// [ "h", "e", "l", "l", "o" ]