add condition to Javascript filter

Question

I want to filter users by name and or lastname. but I´m getting

TypeError: Cannot read property 'toLowerCase' of undefined


const users = [{name: 'john', lastname: "doe"}, {name: 'mary'}]

let searchName = "jo"
let searchLastName = ""

users.filter((user) =>
    user.name
        .toLowerCase()
        .includes(searchName.toLowerCase())

    && user.lastname
        .toLowerCase()
        .includes(searchLastName.toLowerCase())
)

how can I check for lastname only if user has lastname. (i know they always have name)

Does this answer your question? Typescript: Cannot read property 'toLowerCase' of undefined — Christiaan Westerbeek
– Christiaan Westerbeek, Commented May 26, 2020 at 4:51

Sebastian B. · Accepted Answer · 2020-05-25 21:23:31Z

4

Variant A: lastName check only if lastName is set:

users.filter((user) =>
    user.name
        .toLowerCase()
        .includes(searchName.toLowerCase())

    && (
        !user.lastname
        || user.lastname
            .toLowerCase()
            .includes(searchLastName.toLowerCase())
    )
)

Variant B: additional condition that lastName must be set:

users.filter((user) =>
    user.name
        .toLowerCase()
        .includes(searchName.toLowerCase())

    && user.lastname
    && user.lastname
        .toLowerCase()
        .includes(searchLastName.toLowerCase())
    )
)

answered May 25, 2020 at 21:23

Sebastian B.

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3 Comments

Sebastian B. Over a year ago

Whether you want A or B depends on your use case. Both should avoid the TypeError.

Sebastian B. Over a year ago

&& and || expressions evaluate their operands as boolean values, so user.lastName gets converted to a boolean value (true / false) first. As Pac0 wrote, empty strings, null or undefined are converted to false, but other values are also Falsy values

handsome Over a year ago

I will go with option A it works fine with !searchLastName || .... thanks!

Pac0 · Accepted Answer · 2020-05-25 21:22:54Z

3

You can add a check on the existence of the variable with JS automatic boolean coercion :

&& user.lastname   // This will return false if empty, null or undefined
&& user.lastname
    .toLowerCase()
    .includes(searchLastName.toLowerCase())

answered May 25, 2020 at 21:22

Pac0

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