JavaScript Concise tutorial (5) Conditional judgment

JavaScript uses the if () {...} or else {...} To make conditional judgments. For example, depending on age, you can use the if statement to achieve the following:

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var age =; if //if age >= 18 is true, Execute the IF statement block      ' adult ' " else { //Otherwise executes the ELSE statement block       alert ( ' teenager ' }

where else statements are optional. If the statement block contains only one statement, you can omit {}:

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var age =; if      alert ( ' adult ' else      alert ( ' teenager ' );

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

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var age =; if &NBSP;&NBSP;&NBSP;&NBSP; alert ( ' adult ' else &NBSP;&NBSP;&NBSP;&NBSP; console.log ( //add a line of logs       alert ( ' teenager ' Code class= "JavaScript plain"); //<-This line of statements is no longer within the control of else

the Else clause of the above code is actually only responsible for executing console.log (' Age < 18 '); , the original alert (' teenager '); no longer belong to If...else ... control, and it executes every time.

Multi-line condition judgment

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

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var age = 3;if (age >= 18) {    alert(‘adult‘);} else if (age >= 6) {    alert(‘teenager‘);} else {    alert(‘kid‘);}

What if the conditional judgment statement result of the IF is not true or false ? For example:

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var s = ' 123 ' if (s.length) { //condition evaluates to 3      // }

JavaScript treats null,undefined,0,NaN, and empty string ' ' as false, All other values are treated as true, so the result of the above code condition is true.

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JavaScript Concise tutorial (5) Conditional judgment