JavaScript Basics (7) Conditional judgment

JavaScript if () { ... } else { ... } is used to make conditional judgments. For example, depending on the age of different content, you can use the if statement to implement the following:

var age =; if // if age >= 18 is true, the IF statement block    is executed Alert (' adult 'else//  Otherwise execute the ELSE statement block    alert (' Teenager ');}

Where else the statement is optional. If the statement block contains only one statement, you can omit {} :

var age =; if (age >=)    alert (' adult '); Else     alert (' teenager ');

{}the danger of omitting is that if you later want to add some statements and forget to write {} , you change if...else... the semantics, for example:

var age =; if (age >=)    alert (' adult '); Else     console.log (//  Add a line of logs    //  <-This line of statements is no longer in the else's control.

The clause in the above code is else actually only responsible for execution console.log(‘age < 18‘); , the original alert(‘teenager‘); control is not in the if...else... scope, it will execute each time.

That's why we suggest to always write {} .

Multi-line condition judgment

If you want to judge the condition more carefully, you can use multiple if...else... combinations:

var age = 3; if (age >=) {    alert (' adult 'elseif (>= 6) {    alert (' Teenager 'else  {    alert (' Kid ');}

These multiple if...else... combinations are actually equivalent to two layers if...else... :

var age = 3; if (age >=) {    alert (' adult 'else  {    if (>= 6) {        alert (' teenager ');     Else {        alert (' Kid ');}    }

But we usually put else if ligatures together to increase readability. The else slight drop here {} is no problem, because it contains only a single if statement. Note that the last individual does not have to be else omitted {} .

Note that if...else... the execution of the statement is characterized by a two selection, in multiple if...else... statements, if a condition is established, then the subsequent will not continue to judge.

ifWhat if the conditional judgment statement result is not true or false what? For example:

var s = ' 123 '; if // conditional calculation result is 3    //}

JavaScript treats,, null undefined 0 , NaN and empty strings ‘‘ as false other values are treated as such, true so the result of the above code condition judgment is true .

JavaScript Basics (7) Conditional judgment