Conditional statement
When you write code, you often need to perform different behaviors according to different conditions. You can use conditional statements in your code to accomplish this task.
In the web effects, we can use the following conditional statements:
If statement
Executes code when a specified condition is set up.
If...else statement
Executes code when the specified condition is set up, and executes additional code when the condition is not valid.
If...else If....else Statement
Use this statement to choose to execute one of several block of code.
Switch statement
If statement
You can use this statement if you want the specified condition to execute code when it is set up.
Grammar:
if (condition)
{
Execute code when conditions are set
}
Note: Please use lowercase letters. Using uppercase if can be an error!
Instance 1
<script type= "Text/javascript" >
Write a "Good morning" greeting if
The time is less than 10var d=new date ()
var time=d.gethours ()if (time<10)
{
document.write ("<b>good morning</b>")
}
</script> Example 2
<script type= "Text/javascript" >
Write "lunch-time!" if the time is 11var d=new date ()
var time=d.gethours ()if (time==11)
{
document.write ("<b>lunch-time!</b>")
}
</script>
Note: Use the double equal sign (= =) to compare variables!
Note: There is no else in the syntax. The code executes only if the condition is true
First, use the common ternary operator
Copy code code as follows:
<body>
<script type= "Text/javascript" >
var d = new Date ()
var time = d.gethours ()if (Time < 10)
{
document.write ("<b> Good Morning </b>")
}
Else
{
document.write ("<b> Wish you a Happy </b>")
}
</script>
<p> This example demonstrates the If...else statement. </p>
<p> If the browser time is less than 10, then will ask you "Good morning", otherwise will greet you "wish you Happy". </p>
</body>
if (foo) bar (); else Baz (); ==> Foo?bar (): Baz ();
if (!foo) bar (); else Baz (); ==> Foo?baz (): Bar ();
if (foo) return bar (); else return Baz (); ==> return Foo?bar (): Baz ();
For the above use ternary operator to optimize the IF statement you are certainly not unfamiliar, perhaps you often use it.
II. Use and (&&) and OR (| |) Operator
Copy code code as follows:
if (foo) bar (); ==> Foo&&bar ();
if (!foo) bar (); ==> foo| | Bar ();
To be honest, I didn't write code like this, I've seen it when I was studying "brother Bird's Linux private dish", but I didn't expect it to happen in JS.
Three, omit curly braces {}
Copy code code as follows:
if (foo) return bar (); else something (), ==> {if (foo) return bar (); something ()}
You and I are familiar with this type of writing, but I suggest doing it in code optimization, or handing it over to Uglifyjs to help you solve it. After all, less than a curly brace, the code is not very readable.
Writing here, I think of the way in which the father of jquery is getting HTML element attributes in "proficient in JavaScript."
Copy code code as follows:
function GetAttr (el, Attrname) {
var attr = {' for ': ' Htmlfor ', ' class ': ' ClassName '}[attrname] | | Attrname;
};
If we do not write this, we may need to use two if statements for processing, and the above code is not only simple and effective, but also strong readability.
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