Update README.md

Author: yanglbme

solution/0500-0599/0592.Fraction Addition and Subtraction/README.md

@@ -174,6 +174,54 @@ func gcd(a, b int) int {
 }
 ```
 
+#### JavaScript
+
+```js
+/**
+ * @param {string} expression
+ * @return {string}
+ */
+var fractionAddition = function (expression) {
+    let x = 0,
+        y = 1;
+
+    if (!expression.startsWith('-') && !expression.startsWith('+')) {
+        expression = '+' + expression;
+    }
+
+    let i = 0;
+    const n = expression.length;
+
+    while (i < n) {
+        const sign = expression[i] === '-' ? -1 : 1;
+        i++;
+
+        let j = i;
+        while (j < n && expression[j] !== '+' && expression[j] !== '-') {
+            j++;
+        }
+
+        const [a, b] = expression.slice(i, j).split('/').map(Number);
+        x = x * b + sign * a * y;
+        y *= b;
+        i = j;
+    }
+
+    const gcd = (a, b) => {
+        while (b !== 0) {
+            [a, b] = [b, a % b];
+        }
+        return Math.abs(a);
+    };
+
+    const z = gcd(x, y);
+    x = Math.floor(x / z);
+    y = Math.floor(y / z);
+
+    return `${x}/${y}`;
+};
+```
+
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