为什么这个javascript代码有效？
[英]Why does this javascript code work?
问题描述
<!doctype html>
<html lang="en">
<head>
<title>Phrase-o-matic</title>
<meta charset="utf-8">
<style>
body {
font-family: Verdana, Helvetica, sans-serif;
}
</style>
<script>
function makePhrases() {
var words1 = ["24/7", "multi-Tier", "30,000 foot", "B-to-B", "win-win"];
var words2 = ["empowered", "value-added", "oriented", "focused", "aligned"];
var words3 = ["process", "solution", "tipping-point", "strategy", "vision"];
var rand1 = Math.floor(Math.random() * words1.length);
var rand2 = Math.floor(Math.random() * words2.length);
var rand3 = Math.floor(Math.random() * words3.length);
var phrase = words1[rand1] + " " + words2[rand2] + " " + words3[rand3];
var phraseElement = document.getElementById("phrase");
phraseElement.innerHTML = phrase;
}
window.onload = makePhrases;
</script>
</head>
<body>
<h1>Phrase-o-Matic says:</h1>
<p id="phrase"></p>
</body>
</html>
This is an example from a book i am reading on javascript why does the: 
这是我在javascript上阅读的一本书的例子为什么：
var rand1 = Math.floor(Math.random() * words1.length);
var rand2 = Math.floor(Math.random() * words2.length);
var rand3 = Math.floor(Math.random() * words3.length);
generate values that always round down to an index value in the arrays words1 , words2 and words3 ? 生成值总是向下舍入到数组words1 ， words2和words3的索引值？ Why does it never get a value above the last index number of 4 为什么它永远不会得到高于最后一个索引号4的值
4 个解决方案
解决方案1
2 2013-12-18 03:07:10
Math.random() * words1.length means [0, words1.length) . Math.random() * words1.length表示[0, words1.length) 。 So the value always smaller than words1.length . 所以值总是小于words1.length 。
解决方案2
1 2013-12-18 03:08:49
floor() rounds down (always down) to the nearest integer. floor（）向下舍入（总是向下）到最接近的整数。 Math.random() generates a number greater than or equal to 0 and less than 1. Math.random（）生成一个大于或等于0且小于1的数字。
If the length is 4, random() will generate a number between 0 and 3.9999999999, and round down. 如果长度为4，则random（）将生成0到3.9999999999之间的数字，并向下舍入。 So the value will always be a valid index (0 to 3 inclusive) for a 4-element array. 因此，对于4元素数组，该值始终是有效索引（0到3，包括0和3）。
解决方案3
0 2013-12-18 03:08:56
Math.random() returns a value between 0 (inclusive) and 1 (exclusive). Math.random（）返回0（包括）和1（不包括）之间的值。 So you may get for instance 0.1234. 所以你可能会得到0.1234。 You then multiply this by the length of the array which is 4 in this case. 然后将此乘以数组的长度，在这种情况下为4。 The lowest possible value is 0 * 4 = 0 and the highest possible value is 0.9999 * 4 < 4. 最低可能值为0 * 4 = 0，最高可能值为0.9999 * 4 <4。
The floor function truncates the the number leaving only the integer part which will be in the range 0, 1, 2 or 3. floor函数截断数字，只留下整数部分，它将在0,1,2或3范围内。
解决方案4
0 2013-12-18 03:12:11
Math.random() return a float value in [0, 1), which means : Math.random（）在[0,1 ]中返回一个浮点值，表示：
from 0 (inclusive) up to but not including 1 (exclusive)
So Math.random() * 4 will be [0, 4), and Math.floor means: 因此Math.random（）* 4将为[0,4]，而Math.floor意味着：
the largest integer less than or equal to a number
So Math.floor([0,3.xxxx...]) means [0,3] 所以Math.floor（[0,3.xxxx ...]）表示[0,3]
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