How Can I Robustly Check for Nested Key Existence in JavaScript Objects?

Testing for Nested JavaScript Object Key Existence

When dealing with complex JavaScript objects with nested properties, determining the existence of a nested key can be challenging. The following provides an in-depth exploration of the best practices for testing nested object key existence.

The traditional approach, as demonstrated in the question, is to check each level of the nested object sequentially:

if (test.level1 && test.level1.level2 && test.level1.level2.level3) {
    alert(test.level1.level2.level3);
}

However, this approach is prone to exceptions when non-existent properties are encountered.

Solution: Iterative Existence Checker

For improved robustness, a more suitable solution is to create a function that iteratively checks the existence of nested properties without triggering exceptions:

function checkNested(obj, ...args) {
  for (var i = 0; i < args.length; i++) {
    if (!obj || !obj.hasOwnProperty(args[i])) {
      return false;
    }
    obj = obj[args[i]];
  }
  return true;
}

This function takes any number of property names as arguments and returns true if all of them exist in the nested object. For example:

var test = {level1:{level2:{level3:'level3'}} };

checkNested(test, 'level1', 'level2', 'level3'); // true
checkNested(test, 'level1', 'level2', 'foo'); // false

ES6 Solutions

ES6 offers more concise and elegant options for existence checking:

1. Tail-Recursive Function:

function checkNested(obj, level,  ...rest) {
  if (obj === undefined) return false
  if (rest.length == 0 && obj.hasOwnProperty(level)) return true
  return checkNested(obj[level], ...rest)
}

2. Reduce-Based Function:

For retrieving the value of a nested property, you can use this one-line function:

function getNested(obj, ...args) {
  return args.reduce((obj, level) => obj && obj[level], obj)
}

For example:

console.log(getNested(test, 'level1', 'level2', 'level3')); // 'level3'

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