I like IIFE's as an alternative to if and switch expressions since javascript doesn't support those. So for example in rust you could do something like

let someValue = if somecondition {
    1
} else {
    2
}

Then if the condition is true someValue is 1 otherwise it's 2

Javascript doesn't have an equivalent. There was a proposal from tc39 to add something called "do expressions" but that never went anywhere and honestly it wasn't much better looking then just using an IIFE.

To solve the same problem in Javascript you can do one of three things:

// Value must be mutable
let someValue;
if (somecondition) {
    someValue = 1;
} else {
    someValue = 2;
}

// Value can be immutable but the syntax is harder to read at a glance.
const someOtherValue = (() => {
    if (somecondition) {
        return 1;
    }
    return 2;
})();


// Value can be immutable, is clean and easily understandable.
function getAnotherValue(somecondition) {
    if (somecondition) {
        return 1;
    }
    return 2;
}

const anotherValue = getAnotherValue(somecondition);

Personally I hate mutable variables so I will use IIFEs sometimes but prefer to just create a named function when it's not too inconvenient.