StrangeThingsJS

eval-disable-optimization.js

var bar = "bar";

function foo(str) {
	eval(str);
	console.log(bar);
}

foo("var bar = 42;");
// 42

// js engine to not able to optimize the local and global scopes lookup when using eval
// as a result, it will be slower
// setTimeout with passing in a string using eval underneath  

gloabl-variable-leakage.js

function foo() {
  bar = 'baz';
}

foo();
console.log(window.bar);
// 'baz', even though we didn't declare bar in global scope  


function foo2() {
  'use strict';
  bar2 = 'baz';
}

foo2();
// Uncaught ReferenceError: bal is not defined

let-dont-hoist.js

function foo(bar) {
  if(bar) {
    console.log(baz);
    // ReferenceError
    let baz = 'baz';
  }
}

foo(1)

map-with-parseInt.js

['1', '2', '3'].map(parseInt)
// [1, NaN, NaN]

['1', '2', '3'].map(parseFloat)
// [1, 2, 3]

['1', '2', '3'].map(Number)
// [1, 2, 3]

StrangerThingsJS.md

Scope

Lexical Scope

Hoist

this-keyword.js

function foo () {
  console.log(this.bar);
}

var bar = 'bar1';
var a = { bar: 'bar2', foo: foo };
var b = { bar: 'bar3', foo: foo };

foo(); // 'bar1'
a.foo(); // 'bar2'
b.foo(); // 'bar3'