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javascript/snippets/luhn-check.md

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+---
+title: Luhn check
+type: snippet
+tags: [math,algorithm]
+cover: blank-card
+excerpt: Implements the Luhn Algorithm, used to validate a variety of identification numbers.
+dateModified: 2022-01-30T13:37:39+02:00
+---
+
+Implements the [Luhn Algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers etc.
+
+- Use `String.prototype.split()`, `Array.prototype.reverse()` and `Array.prototype.map()` in combination with `parseInt()` to obtain an array of digits.
+- Use `Array.prototype.shift()` to obtain the last digit.
+- Use `Array.prototype.reduce()` to implement the Luhn Algorithm.
+- Return `true` if `sum` is divisible by `10`, `false` otherwise.
+
+```js
+const luhnCheck = num => {
+  const arr = (num + '')
+    .split('')
+    .reverse()
+    .map(x => parseInt(x));
+  const lastDigit = arr.shift();
+  let sum = arr.reduce(
+    (acc, val, i) => (i % 2 !== 0 ? acc + val : acc + ((val *= 2) > 9 ? val - 9 : val)),
+    0
+  );
+  sum += lastDigit;
+  return sum % 10 === 0;
+};
+```
+
+```js
+luhnCheck('4485275742308327'); // true
+luhnCheck(6011329933655299); //  true
+luhnCheck(123456789); // false
+```