Hướng dẫn dùng exampleo JavaScript
Loops can execute a block of code a number of times. Show Nội dung chính
Nội dung chính
JavaScript LoopsLoops are handy, if you want to run the same code over and over again, each time with a different value. Often this is the case when working with arrays: Instead of writing: text += cars[0] + " You can write: for (let i = 0; i < cars.length; i++)
{ Try it Yourself » Different Kinds of LoopsJavaScript supports different kinds of loops:
The For LoopThe for (expression 1; expression 2; expression 3) { Expression 1 is executed (one time) before the execution of the code block. Expression 2 defines the condition for executing the code block. Expression 3 is executed (every time) after the code block has been executed. Example for (let i = 0; i < 5; i++) { Try it Yourself » From the example above, you can read: Expression 1 sets a variable before the loop starts (let i = 0). Expression 2 defines the condition for the loop to run (i must be less than 5). Expression 3 increases a value (i++) each time the code block in the loop has been executed. Expression 1Normally you will use expression 1 to initialize the variable used in the loop (let i = 0). This is not always the case, JavaScript doesn't care. Expression 1 is optional. You can initiate many values in expression 1 (separated by comma): Example for (let i = 0, len = cars.length, text = ""; i < len; i++) { Try it Yourself » And you can omit expression 1 (like when your values are set before the loop starts): Example let i = 2; Try it Yourself » Expression 2Often expression 2 is used to evaluate the condition of the initial variable. This is not always the case, JavaScript doesn't care. Expression 2 is also optional. If expression 2 returns true, the loop will start over again, if it returns false, the loop will end. If you omit expression 2, you must provide a break inside the loop. Otherwise the loop will never end. This will crash your browser. Read about breaks in a later chapter of this tutorial. Expression 3Often expression 3 increments the value of the initial variable. This is not always the case, JavaScript doesn't care, and expression 3 is optional. Expression 3 can do anything like negative increment (i--), positive increment (i = i + 15), or anything else. Expression 3 can also be omitted (like when you increment your values inside the loop): Example let i = 0; Try it Yourself » Loop ScopeUsing Examplevar i = 5; for (var i = 0; i < 10; i++) { // Here i is 10 Try it Yourself » Using Examplelet i = 5; for (let i = 0; i < 10; i++) { // Here i is 5 Try it Yourself » In the first example, using In the second example, using When For/Of and For/In LoopsThe While LoopsThe |