30 lines
1.4 KiB
Markdown
30 lines
1.4 KiB
Markdown
The task demonstrates how postfix/prefix forms can lead to different results when used in comparisons.
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1. **From 1 to 4**
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```js run
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let i = 0;
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while (++i < 5) alert( i );
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```
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The first value is `i=1`, because `++i` first increments `i` and then returns the new value. So the first comparison is `1 < 5` and the `alert` shows `1`.
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Then follow `2,3,4…` -- the values show up one after another. The comparison always uses the incremented value, because `++` is before the variable.
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Finally, `i=4` is incremented to `5`, the comparison `while(5 < 5)` fails, and the loop stops. So `5` is not shown.
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2. **From 1 to 5**
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```js run
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let i = 0;
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while (i++ < 5) alert( i );
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```
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The first value is again `i=1`. The postfix form of `i++` increments `i` and then returns the *old* value, so the comparison `i++ < 5` will use `i=0` (contrary to `++i < 5`).
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But the `alert` call is separate. It's another statement which executes after the increment and the comparison. So it gets the current `i=1`.
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Then follow `2,3,4…`
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Let's stop on `i=4`. The prefix form `++i` would increment it and use `5` in the comparison. But here we have the postfix form `i++`. So it increments `i` to `5`, but returns the old value. Hence the comparison is actually `while(4 < 5)` -- true, and the control goes on to `alert`.
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The value `i=5` is the last one, because on the next step `while(5 < 5)` is false.
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