This expression is not callable. Type 'string' has no call signatures - node.js

Question

I'm trying to create a template file from an object where the key can be either a string or a function that returns a string:
export const createDynamicTemplate = (
templateParams: CreateDynamicTemplateParams
) => {
const { template, projectPath = '', param = '' } = templateParams
const updatedTemplateArr = Object.keys(template).map((key: string) => {
return {
[key]: {
filePath: `${projectPath}/${key}`,
template: typeof template[key] === 'function' ?
template[key](param) : template[key],
},
}
})
const updatedTemplate = Object.assign({}, ...updatedTemplateArr)
return updatedTemplate
}
My interfaces are:
export interface TemplateObject {
[key: string]: string
}
export interface FunctionalTemplateObject {
[key: string]: (param: string) => void
}
export interface CreateDynamicTemplateParams {
template: FunctionalTemplateObject | TemplateObject
projectPath: string
param: string
}
It keeps throwing this error in createDynamicTemplate though:
This expression is not callable.
Not all constituents of type 'string | ((param: string) => void)' are callable.
Type 'string' has no call signatures.
What am I doing wrong here?

Answer 1

Checking the type of a child property of a variable which references an object will not narrow the type of the variable-object. You can save the value at the key in a separate variable first, then check that value to narrow its type:
const updatedTemplateArr = Object.keys(template).map((key: string) => {
const item = template[key];
if (typeof item === 'function') {
return {
[key]: {
filePath: `${projectPath}/${key}`,
template: item(param),
},
}
}
})
Or, even better, use Object.entries to get the key and value at once. (Also note that there's no need to note the type of the .map parameters - TS can infer it automatically just fine)
const updatedTemplateArr = Object.entries(template).map(([key, value]) => {
if (typeof value === 'function') {
return {
[key]: {
filePath: `${projectPath}/${key}`,
template: value(param),
},
}
}
})