Optional Chaining

October 23, 2020 ยท 2 min read

A common error in the console I've seen is Error: Cannot read property '<name>' of undefined where <name> is a value expected to be other than undefined or null.

Here's an example of a player object that has a collection of Pokemon. Each Pokemon object has at least a name, Pokedex ID, and their pre-evolution name. What some also have is an evolution property which contains what stone it needs to become this Pokemon from its preEvolution stage.

const player = {
    name: "Dana",
    pokemon: [
        {
            name: "Lucario",
            dexId: 448,
            preEvolution: "Riolu"
        },
        {
            name: "Roserade",
            dexId: 407,
            evolution: {
                stone: "Shiny"
            },
            preEvolution: "Roselia"
        },
        {
            name: "Beautifly",
            dexId: 267,
            preEvolution: "Silcoon"
        },
        {
            name: "Ampharos",
            dexId: 181,
            preEvolution: "Flaaffy"
        },
        {
            name: "Flareon",
            dexId: 136,
            evolution: {
                stone: "Fire"
            },
            preEvolution: "Eevee"
        },
        {
            name: "Froslass",
            dexId: 478,
            evolution: {
                stone: "Dawn"
            },
            preEvolution: "Snorunt"
        },
    ]
}

If I wanted to print all the Pokemon names to the console, I can do:

player.pokemon.forEach(pokemon => console.log(pokemon.name));

But what if I want to print out any evolution stones that the pokemon requires to evolve?

This code would give an error since not all pokemon have one:

player.pokemon.forEach(pokemon => console.log(pokemon.evolution.stone));

// TypeError: Cannot read property 'stone' of undefined

You could do a check by adding an if-statement if the pokemon has an evolution property:

player.pokemon.forEach(pokemon => {
    if(pokemon.evolution !== undefined){
        console.log(pokemon.evolution.stone)
    }
    else {
        console.log(undefined);
    }
});

// undefined
// "Shiny"
// undefined
// undefined
// "Fire"
// "Dawn"

While this works, there's a shorthand property called "optional chaining" using ?. to check if the property is undefined or null before continuing with the expression:

player.pokemon.forEach(pokemon => {
   console.log(pokemon.evolution?.stone)
});

// undefined
// "Shiny"
// undefined
// undefined
// "Fire"
// "Dawn"

This syntax isn't only used on objects. It can be put after any variable, Array, or Function as a check.

To execute a Function that may not be defined, you can append () after the optional chaining to execute it.

If we weren't sure if the player object had a method called possibleFunction to use, we can check it before executing it.

player.possibleFunction?.()

I find myself using this shorthand approach more often than the usual if-statement to see if a value is undefined or null to avoid an error.

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