feat: add javascript solution to lc problem: No.1881.Maximum Value after Insertion (doocs#420)

Author: zhaocchen

solution/1800-1899/1881.Maximum Value after Insertion/README.md

@@ -108,6 +108,29 @@ class Solution {
 }
 ```
 
+### **JavaScript**
+
+```js
+/**
+ * @param {string} n
+ * @param {number} x
+ * @return {string}
+ */
+ var maxValue = function(n, x) {
+    let nums = [...n];
+    let sign = 1, i = 0;
+    if (nums[0] == '-') {
+        sign = -1;
+        i++;
+    }
+    while (i < n.length && (nums[i] - x) * sign >= 0) {
+        i++;
+    }
+    nums.splice(i, 0, x);
+    return nums.join('');
+};
+```
+
 ### **...**
 
 ```

solution/1800-1899/1881.Maximum Value after Insertion/README_EN.md

@@ -98,6 +98,29 @@ class Solution {
 }
 ```
 
+### **JavaScript**
+
+```js
+/**
+ * @param {string} n
+ * @param {number} x
+ * @return {string}
+ */
+ var maxValue = function(n, x) {
+    let nums = [...n];
+    let sign = 1, i = 0;
+    if (nums[0] == '-') {
+        sign = -1;
+        i++;
+    }
+    while (i < n.length && (nums[i] - x) * sign >= 0) {
+        i++;
+    }
+    nums.splice(i, 0, x);
+    return nums.join('');
+};
+```
+
 ### **...**
 
 ```

solution/1800-1899/1881.Maximum Value after Insertion/Solution.js

@@ -0,0 +1,18 @@
+/**
+ * @param {string} n
+ * @param {number} x
+ * @return {string}
+ */
+ var maxValue = function(n, x) {
+    let nums = [...n];
+    let sign = 1, i = 0;
+    if (nums[0] == '-') {
+        sign = -1;
+        i++;
+    }
+    while (i < n.length && (nums[i] - x) * sign >= 0) {
+        i++;
+    }
+    nums.splice(i, 0, x);
+    return nums.join('');
+};