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<article>
<h2 id="10-2">10-2 Exponentiation Operator (指數運算子)</h2>
<h3>ES6之前的 Math.pow()</h3>
<p>
過去要進行指數運算 (即計算 a 的 n 次方 , a 為底數， n
為指數，通常指數寫成上標，放在底數的右邊)，常用的是 Math.pow()
</p>
<pre><code class="language-js">
// 平方：
let x = 2;
console.log(Math.pow(x, 2)); // 4
// 相當於
console.log(x * x); // 4
</code></pre>
<pre><code class="language-js">
// 三次方：
let x = 2;
console.log(Math.pow(x, 3)); // 8
// 相當於
console.log(x * x * x); // 8
</code></pre>
<pre><code class="language-js">
// 函式表示
let square = x => Math.pow(x, 2);
let cube = x => Math.pow(x, 3);
console.log(square(6)); // 36
console.log(cube(3)); // 27
</code></pre>
<h3>現代的 Exponentiation Operator</h3>
<p>在 ES2016 (ES7) 提供了 exponentiation operator (指數運算子)，用於指數運算。</p>
<p>exponentiation 運算子會先做 Number()強轉。基本運算範例如下:</p>
<pre><code class="language-js">
2 ** 3; // 8
3 ** 2; // 9
3 ** 2.5; // 15.588457268119896
10 ** -1; // 0.1
2 ** 1024; // Infinity
Infinity ** Infinity // Infinity
NaN ** 2; // NaN
1 ** Infinity; // NaN
undefined ** 2 // NaN
NaN ** NaN // NaN
NaN ** 0; // 1
0 ** 0 // 1
undefined ** 0 // 1
'' ** '' // 1
0 ** 2 // 0
null ** 2 // 0
2n ** 3n; // 8n
2n ** 1024n; // A very large number, but not Infinity
2n ** 2; // TypeError: Cannot mix BigInt and other types, use explicit conversions
// To do exponentiation with a BigInt and a non-BigInt, convert either operand
2n ** BigInt(2); // 4n
Number(2n) ** 2; // 4
</code></pre>
<h3>Associativity</h3>
<p>exponentiation 運算子的 associativity 是 right-associative</p>
<pre><code class="language-js">
console.log(2 ** 3 ** 2); // 512
console.log(2 ** (3 ** 2)); // 512
// 實際計算順序：
// 1. 2 ** 3 ** 2
// 2. 2 ** 9
// 3. 512
</code></pre>
<p>若加上括號則結果會不同</p>
<pre><code class="language-js">
console.log((2 ** 3) ** 2); // 64
// 實際計算順序：
// 1. (2 ** 3) ** 2
// 2. 8 ** 2
// 3. 64
</code></pre>
<h3>Usage with unary operators</h3>
<p>搭配其他運算子使用錯誤案例如下:</p>
<pre><code class="language-js">
let a = 2;
let n = 3;
console.log(+a ** n);
console.log(-a ** n);
console.log(~a ** n);
console.log(!a ** n);
delete a ** n;
void a ** n;
typeof a ** n;
// 以上全部 SyntaxError: Unary operator used immediately before exponentiation expression. Parenthesis must be used to disambiguate operator precedence
</code></pre>
<p>
由此可知不能直接加在前面‧我們看一下
<a
href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation"
>
mozilla Exponentiation
</a>
給出的正確範例:
</p>
<pre><code class="language-js">
-(2 ** 2); // -4
</code></pre>
<pre><code class="language-js">
(-2) ** 2; // 4
</code></pre>
<h3>Exponentiation assignment (**=)</h3>
<pre><code class="language-js">
let a = 3;
console.log(a **= 2);
// Expected output: 9
console.log(a **= 0);
// Expected output: 1
console.log(a **= 'hello');
// Expected output: NaN
</code></pre>
<pre><code class="language-js">
let bar = 5;
bar **= 2; // 25
bar **= "foo"; // NaN
</code></pre>
<pre><code class="language-js">
// 可以計算 BigInt
let foo = 3n;
foo **= 2n; // 9n
// 比較 ES6 以前處理 BigInt 的寫法是會報錯
console.log(Math.pow(2n, 3n)); // TypeError: Cannot convert a BigInt value to a number
</code></pre>
<p>
參考資料:
<a
href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Exponentiation_assignment"
>
Exponentiation assignment (**=)n
</a>
</p>
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