CFA issue

[normalser]

It's probably duplicate but it's not easy to find so apologies if it is

interface A {
    a: string // or `a: 'a'`
}

interface B {
    a: number
}

declare var c:A|B

if (typeof c.a == 'string') { // or `if (c.a == 'a')`
    c // <- Type: A|B, should be A ?
}
else {
    c // <- Type: A|B, should be B ?
}

// where this seems ok:

declare var d:string|number // or `'a'|number`

if (typeof d == 'string') { // or `if (d == 'a')`
    d // Type: string           // Type: 'a'
}
else {
    d // Type: number
}

[HolgerJeromin]

Adding a User-Defined Type Guards should solve your problem.

[RyanCavanaugh]

I'd be interested to see what the un-reduced version of this code looked like

[normalser]

@RyanCavanaugh I updated above with comments cause I initially experienced it with string vs number but played with some other cases like string literal types vs number

Here is the un-reduced but still simplified example of the real code:

import * as React from "react";

interface StringField {
    data: string
    maxLength: number
}

interface NumberField {
    data: number
    maxValue: number
}

type FormField = StringField | NumberField

interface Props {
    fields: FormField[]
}

export class FormComponent extends React.Component<Props, {}> {
    render() {
        const { fields } = this.props
        return (
            <div>
                {fields.map(field=>
                    <div>
                        {typeof(field.data) == 'string' && <div>TODO: Render component for string: {field.data} with max length: {field.maxLength}</div>}
                         // Error here because field's type is still StringField | NumberField
                        {typeof(field.data) == 'number' && <div>TODO: Render field for number: {field.data} with max value: {field.maxValue}</div>}
                         // Error here because field's type is still StringField | NumberField
                    </div>)}
            </div>
        );
    }
}

I know there are workarounds - but would be fantastic if TS could handle such cases as well. Thanks

[RyanCavanaugh]

Closing due to minimal feedback; people don't seem to be encountering this very often