Catching unknown keys in lookup tables
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Posted on The lookup table pattern can replace some long if-else branches and switch statements. We can catch unknown keys we don’t have values for with hasOwnProperty.
const getColorOf = (thing) => {
const map = {
clouds: "white",
grass: "green",
sky: "blue",
}
// throw an error if `thing` isn’t a key in the map
if (!map.hasOwnProperty(thing)) {
throw new Error(`Color not defined for: ${thing}`)
}
// if we get here, `thing` IS a key in the map
return map[thing]
}
getColorOf("grass") // ⇒ "green"
getColorOf("air") // ⇒ Color not defined for: air
getColorOf() // ⇒ Color not defined for: undefinedconst getColorOf = (thing) => {
const map = {
clouds: "white",
grass: "green",
sky: "blue",
}
// throw an error if `thing` isn’t a key in the map
if (!map.hasOwnProperty(thing)) {
throw new Error(`Color not defined for: ${thing}`)
}
// if we get here, `thing` IS a key in the map
return map[thing]
}
getColorOf("grass") // ⇒ "green"
getColorOf("air") // ⇒ Color not defined for: air
getColorOf() // ⇒ Color not defined for: undefinedconst getColorOf = (thing) => {
const map = {
clouds: "white",
grass: "green",
sky: "blue",
}
// throw an error if `thing` isn’t a key in the map
if (!map.hasOwnProperty(thing)) {
throw new Error(`Color not defined for: ${thing}`)
}
// if we get here, `thing` IS a key in the map
return map[thing]
}
getColorOf("grass") // ⇒ "green"
getColorOf("air") // ⇒ Color not defined for: air
getColorOf() // ⇒ Color not defined for: undefinedconst getColorOf = (thing) => {
const map = {
clouds: "white",
grass: "green",
sky: "blue",
}
// throw an error if `thing` isn’t a key in the map
if (!map.hasOwnProperty(thing)) {
throw new Error(`Color not defined for: ${thing}`)
}
// if we get here, `thing` IS a key in the map
return map[thing]
}
getColorOf("grass") // ⇒ "green"
getColorOf("air") // ⇒ Color not defined for: air
getColorOf() // ⇒ Color not defined for: undefined