hoisting

问题 I asked a question before and somebody give me a guide and I read it and I saw this var temp = setTimeout, setTimeout = function() {}; He said that temp will be undefined due to JavaScript hoisting and I dont understand why Its not should be like that? var temp; temp = setTimeout; setTimeout = function() {}; so why its undefined? 回答1: This is not the same. Your multiple var declaration also declares setTimeout : var temp = setTimeout, setTimeout = function() {}; which is hoisted to var temp;

问题 I came across a interesting quiz function bar() { return foo; foo = 10; function foo() {} var foo = '11'; } alert(typeof bar()); My interpretation is like this (Which is wrong according to console :) ): var foo; // global variable function bar(){ function foo(){} var foo; // Here variable foo should override foo function return foo; // (according to me foo should be variable with undefined value) What is going on here, How JavaScript resolve naming order ? foo = 10; foo = "11"; } Here is a

问题 Searching for the JSLint error "was used before it was defined" i've found these: JSLint: Using a function before it's defined error Function was used before it was defined - JSLint JSLint: was used before it was defined jsLint error: “somefunction() was used before it was defined” jslint - Should we tolerate misordered definitions? Problem None of those answers WHY the error is shown. Elaboration According to the ECMA-262 Specification functions are evaluated before execution starts, hence

问题 I've been learning a lot of Javascript lately and I've been trying to understand the value (if there is any) of hoisting variables. I understand (now) that JS is a two pass system, it compiles and then executes. Also, I understand that the var keyword 'exists' in the lexical scope it was declared, hence why it's 'undefined' if it's called before it's assigned a value by the engine. The question is, why does that even matter? What use is there to hoisting variables that you can't do without

问题 I've been learning a lot of Javascript lately and I've been trying to understand the value (if there is any) of hoisting variables. I understand (now) that JS is a two pass system, it compiles and then executes. Also, I understand that the var keyword 'exists' in the lexical scope it was declared, hence why it's 'undefined' if it's called before it's assigned a value by the engine. The question is, why does that even matter? What use is there to hoisting variables that you can't do without

问题 I have this code: (function() { var ex; try { throw new Error('blah'); } catch(ex) { console.log('ex i here:', ex); } console.log('ex out here:', ex); return 'hi'; })() This logs: ex i here: Error('blah'); ex out here: undefined Why is this so? I would think due to hoisting, ex would get set outside of this block scope, so it should be available in ex out here . I expected it to work similar to a for loop: for (i=0; i<2; i++) { } console.log(i); // gives 2 回答1: You are messing few things up.

问题 david sharif made a JS quiz which pretty much looks like- var foo=1; function bar(){ return foo; foo=10; function foo(){} var foo =5; } typeof bar();//? In my understanding, functions are hosited first and then variable declared inside. the hosited form of the function would be something like (correct me if i am wrong)- var foo=1; function bar(){ function foo(){} var foo; return foo; foo=10; foo =5; } typeof bar();//? why typeof bar() is function not undefined? Is this because of, at the time

问题 I've noticed some scripts seem to be called before others on a certain page, I was wondering, what is the specific order for scripts to load? In-page before referenced .js scripts? Are they run in order from first <script> mentioned to last in page, or Is this browser-dependent? How can one make sure that a specific script is first to run in a page? 回答1: I've noticed some scripts seem to be called before others on a certain page. I was wondering, what is the specific order for scripts to load

问题 Running this in Chrome and Firefox gives different answers: (function() { if(true) { function f() { alert("yes"); }; } else { function f() { alert("no"); }; } f(); })(); In Chrome the result is 'no' In Firefox the result is 'yes' Why the difference? 回答1: Declaring functions inside conditional statements is non-standard, so do not do that . That's a known issue. You may use function expressions instead of the declarations: var f; if(true) { f = function() { alert("yes"); }; } else { f =

问题 I've been reading about JavaScript hoisting sometime back. JavaScript Scoping and Hoisting by Ben Cherry Two words about “hoisting” by Dmitry Soshnikov and, some more about JavaScript type-coercion, truth & false test: Truth, Equality and JavaScript and some other resource And while practicing some, and found I m missing some important concept about the hoisting and a variable' truthy & falsy. 1: 'if' truth test with duplicate variable declaration var foo = 1; function bar() { if (!foo) {