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There are four ways to write switch, you know
2022-07-01 20:28:00 【51CTO】
JavaScript Of switch There are four ways to write , Do you know? ？ Whether you know it or not , Anyway, I don't know .
What I know JavaScript Of switch There is only one way to write a sentence . But when it comes to dealing with branches , There are many ways to write .if The branch writing method can be regarded as one ,switch Branch writing can be regarded as the second , The third is to use the strategy mode , If you want to count conditional operators , Um. , Just four .
But the main character of this article is switch. Everyone knows switch Generally speaking, the way of writing is switch Variable or expression ,case Constant . Um. , for instance , A hundred mark mark ,90 And 90 A score above is excellent ,80 And above 90 The following is good ,60 And above 80 The following is qualified ,60 The following are unqualified , use switch I'll probably say that ：
In the code
score / 10 | 0 and Math.floor(score / 10) It's the same effect , Is divided by 10 Take the integer part of the quotient .
This paragraph switch Well done , Use rounding to avoid using a long string if ... else Branching is also a coincidence .
But now the rules have changed , Separate qualified and good points from 80 The score dropped to 75 branch , What should I do ？
The rounding method above is still ok , But this time the divisor is no longer 10, It is 5. Accordingly ,case A lot more ：
- 18、19、20 It's excellent
- 15、16、17 Is good
- 12、13、14 It's qualified
- The rest is unqualified
Write 9 individual case, It's better to use if ... else Forget it .
Is it? ？ Actually use switch There are also simpler ways to write ：
Does it feel strange ？ This is not used to at all switch expression case Constant , It's the opposite ,switch Constant case expression ！ If you take this program to run , There will be no problem at all . because ——switch and case Is in accordance with the === To match , It doesn't care whether it's an expression or a constant , Or say ,switch and case Expressions can be followed ！
Yes , expression ！
So in the example above , hold switch(true) Change to switch( 2 > 1) The same effect .
All right. , Brain hole is open .switch It doesn't matter how many ways to write . The next thing to study is switch Variants ——
notice C# Yes switch expression , be envious , Can it be realized? ？
Don't be greedy ,JavaScript Everything in can be an expression …… If not , use IIFE Just encapsulate one
Pay attention to the score As IIFE Parameters of , Because in actual use , What you may need to pass in is an expression . In this case, it should be evaluated in advance , And only once （ Avoid the side effects of substitution ）.
However, such encapsulation is obviously meaningless , If you really want to package like this , It's better to seal it into a strategy ：
Every strategy contains tester (t) And the value (v) The object of .tester Is a judgement function , Pass in the value to be judged , That is to say switch ( expression ) Here the expression , And this expression is evaluated in advance as IIFE Is passed in . The process of applying strategies is simple and crude , Is to find the first qualified strategy , Take its value .
Of course, this strategy is a little overqualified . Situations that really need strategies , There is usually not a value in the policy , It's an act , That is, function .
We know that switch In the sentence , each case Are in the same scope , So it can't be in two case Statement to declare the same local variable . Although with { } Packages can solve these problems , But the code doesn't look very good , In particular, be careful not to forget break. If you use strategy , It may look pleasant , And don't worry about it break The problem of ：
Here's a demonstration , Results will be output first in strategic behavior , Return to level .
function
calcGrade(
score) {
return ((
value,
rules)
=>
rules.
find(({
t })
=>
t(
value)).
fn(
value))(
score,
[
{
t:
n
=>
n
>=
90,
fn:
score
=> {
const
grade
=
" good ";
console.
log(
grade,
score);
return
grade;
}
},
{
t:
n
=>
n
>=
75,
fn:
score
=> {
const
grade
=
" good ";
console.
log(
grade,
score);
return
grade;
}
},
{
t:
n
=>
n
>=
60,
fn:
score
=> {
const
grade
=
" qualified ";
console.
log(
grade,
score);
return
grade;
}
},
{
t: ()
=>
true,
fn:
score
=> {
const
grade
=
" unqualified ";
console.
log(
grade,
score);
return
grade;
}
},
]
);
}
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The code is really a little long , Because there is strategic behavior logic in it . If you really want to be switch expression To use , The policy part should be an expression , It won't be long . In the above code , Strategy behavior is similar , Can be encapsulated into a function , In this way, it can be written in the form of an expression ：
function
calcGrade(
score) {
const
printGrade
= (
grade,
score)
=> {
console.
log(
grade,
score);
return
grade;
};
return ((
value,
rules)
=>
rules.
find(({
t })
=>
t(
value)).
fn(
value))(
score,
[
{
t:
n
=>
n
>=
90,
fn:
score
=>
printGrade(
" good ",
score) },
{
t:
n
=>
n
>=
75,
fn:
score
=>
printGrade(
" good ",
score) },
{
t:
n
=>
n
>=
60,
fn:
score
=>
printGrade(
" qualified ",
score) },
{
t: ()
=>
true,
fn:
score
=>
printGrade(
" unqualified ",
score) },
]
);
}
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Does it look decent now ？
The code above has different forms , It's all about the same thing , There is no comparison between who is good and who is bad . It looks good, but it's elegant , I don't like it. I'm not favored . In different circumstances , Just choose the right approach . The code above uses find() To find strategies , If you switch to filter(), That would be another scene ~(～￣▽￣)～.
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