This should be foolproof:

const a = {
  a: [{
    b: [{
      prop: 10
    }]
  }]
};

console.log(a?.a?.[0]?.b?.[0]?.prop);

Update 2020

This long-wished feature is now available in JavaScript!

I'll redirect to Gibolt's answer, which covers it well.

Original 2018 answer

• There is no "null-safe navigation operator" in Javascript (EcmaScript 5 or 6), like ?. in C#, Angular templates, etc. (also sometimes called Elvis operator, when written ?:) , at least yet, unfortunately.

• You can test for null and return some dependent expression in a single line with the ternary operator ?:, as already given in other answers :

(use === null to check only for nulls values, and == null to check for null and undefined)

    console.log(myVar == null ? myVar.myProp : 'fallBackValue');

• in some cases, like yours, when your variable is supposed to hold an object, you can simply use the fact that any object is truthy whereas null and undefined are falsy values :

    if (myVar) 
        console.log(myVar.myProp)
    else
        console.log('fallbackValue')
  

  You can test for falsy values by coalescing to boolean with !! and make this inline :

    console.log(!!myVar ? myVar.myProp : 'fallbackValue');
  

  Be very careful though with this "falsy test", for if your variable is 0, '', or NaN, then it is falsy as well, even though it is not null/undefined.

Js logical operators return not true or false, but truly or falsy value itself. For example in expression x && y, if x is falsy, then it will be returned, otherwise y will be returned. So the truth table for operator is correct.

In your case you could use expression customers && customers.orders && customers.orders.Length to get length value or the first falsy one.

Also you can do some magic like ((customers || {}).orders || {}).length (Personally, I don't like this syntax and possible garbage collection pressure as well)

Or even use maybe monad.

Copyfunction Option(value) {
    this.value = value;
    this.hasValue = !!value;
}

Option.prototype.map = function(s) {
    return this.hasValue
        ? new Option(this.value[s])
        : this;
}

Option.prototype.valueOrNull = function() {
    return this.hasValue ? this.value : null;
}

var length = 
    new Option(customers)
        .map("orders")
        .map("length")
        .valueOrNull();

It's longer than all the previous approaches, but clearly shows your intentions without any magic behind.