# indexOf 和 includes # 區別 - indexOf返回的是數值型的而includes返回的是布爾型的 - 都可以支持第二參數，而且的第二個參數都支持負數形式 - 數組中的indexOf不能判斷數組中是否有NaN而includes可以 ### 判斷稀疏數組結果不同 ``` <pre class="calibre13">``` <span class="token2">var</span> ary <span class="token">=</span> <span class="token1">[</span><span class="token1">,</span><span class="token1">,</span><span class="token1">]</span><span class="token1">;</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>ary<span class="token1">.</span><span class="token3">indexOf</span><span class="token1">(</span>undefined<span class="token1">)</span><span class="token1">)</span><span class="token6">//-1</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>ary<span class="token1">.</span><span class="token3">includes</span><span class="token1">(</span>undefined<span class="token1">)</span><span class="token1">)</span><span class="token6">//true</span> ``` ``` 這是因為`indexOf`認為稀疏數組，省略掉的值是不存在的，但`includes`認為是undefined # 字符串的indexOf和數組中的indexOf的比較 1. 這兩個方法都可以接收兩個參數 2. 這兩個方法在沒有查找的指定的字符都返回-1 3. 字符串中的indexOf中的第二個參數不支持負數而數組的indexOf支持 4. 字符串的indexOf在傳入參數不是字符串的情況下默認會轉換為字符串而數組的indexOf不會進行數據類的轉換 字符串的參數不支持負數 ``` <pre class="calibre13">``` <span class="token2">let</span> str <span class="token">=</span> <span class="token4">"abcd"</span><span class="token1">;</span> <span class="token2">let</span> ary <span class="token">=</span> <span class="token1">[</span><span class="token4">"a"</span><span class="token1">,</span><span class="token4">"b"</span><span class="token1">,</span><span class="token4">"c"</span><span class="token1">,</span><span class="token4">"d"</span><span class="token1">]</span><span class="token1">;</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>str<span class="token1">.</span><span class="token3">indexOf</span><span class="token1">(</span><span class="token4">"a"</span><span class="token1">,</span> <span class="token">-</span><span class="token5">1</span><span class="token1">)</span><span class="token1">)</span><span class="token1">;</span> <span class="token6">//0</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>ary<span class="token1">.</span><span class="token3">indexOf</span><span class="token1">(</span><span class="token4">"a"</span><span class="token1">,</span> <span class="token">-</span><span class="token5">1</span><span class="token1">)</span><span class="token1">)</span><span class="token1">;</span> <span class="token6">//-1</span> <span class="token5">1</span> <span class="token5">2</span> <span class="token5">3</span> <span class="token5">4</span> ``` ``` 字符串的參數會自動轉換 ``` <pre class="calibre13">``` <span class="token2">let</span> str <span class="token">=</span> <span class="token4">"1"</span><span class="token1">;</span> <span class="token2">let</span> ary <span class="token">=</span> <span class="token1">[</span><span class="token4">"1"</span><span class="token1">]</span><span class="token1">;</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>str<span class="token1">.</span><span class="token3">indexOf</span><span class="token1">(</span><span class="token5">1</span><span class="token1">)</span><span class="token1">)</span><span class="token1">;</span> <span class="token6">//0</span> console<span class="token1">.</span><span class="token3">log</span><span class="token1">(</span>ary<span class="token1">.</span><span class="token3">indexOf</span><span class="token1">(</span><span class="token5">1</span><span class="token1">)</span><span class="token1">)</span><span class="token1">;</span> <span class="token6">//-1</span> ``` ``` ### 字符串的slice和數組中的slice的比較 1. 在不傳入任何參數的情況下都是把原來的值拷貝一份 2. 字符串的slice的第二個參數是不支持負數的而數組的可以 3. 都可以接收兩個參數 ### 字符串的concat和數組中的concat的比較 1. 這兩個方法都沒有限定參數，而且參數可以是任何數據類型的值