Getting error Uncaught TypeError: Cannot read property 'toLowerCase' of undefined

Question

Getting error Uncaught TypeError: Cannot read property 'toLowerCase' of undefined while executing this code.

 const map  = (array, callback) => {
    return [callback(array[0])].concat(map(array.slice(1),callback));
}
var lower = map(['A','B','C'],function (val) {
    return val.toLowerCase()
});
console.log(lower);

Answer 1

You need to check if array length is 1 then only return the element because there are no other elements left.

 const map = (array, callback) => { if(array.length === 1) return [callback(array[0])] return [callback(array[0])].concat(map(array.slice(1),callback)); } var lower = map(['A','B','C'],function (val) { return val.toLowerCase() }); console.log(lower); 

It seems that you want to create your own map() . In think you loop is better than recursion.

 const map = (array, callback) => { let res = [] for(let i = 0;i<array.length;i++){ res[i] = callback(array[i],i) } return res; } var lower = map(['A','B','C'],function (val) { return val.toLowerCase() }); console.log(lower); 

Why OP's code donot work?

The code in question uses recursion. And whenever we use recursive approach we should have a condition to return the function with calling it again.

• Now consider then the array length is 1
• then array.slice(1) will return []
• an empty array will be passed to map() .
• For an empty array array[0] will `undefined
• then undefined will be passed to callback
• undefined.toLowerCase throws error.

Conclusion

You don't need to run the part .concat(map(array.slice(1),callback)) when only one element is left in array. Because there is noting more to concat()