I'm trying to find a way to check whether a function parameter is an array or not. If it is not, turn it into an array and perform a function on it, otherwise just perform a function on it.

Example:

interface employee {
    first: string,
    last: string
}

function updateEmployees (emp: employee | employee[]) {
    let employees = [];
    if (emp instanceof Array) employees = [emp];
    else employees = emp;
    employees.forEach(function(e){
        return 'something'
    })
}

This seems like it would work to me but is throwing a warning of Type 'employee' is not assignable to type 'any[]'. Property 'length' is missing in type 'employee'.

I'm trying to find a way to check whether a function parameter is an array or not. If it is not, turn it into an array and perform a function on it, otherwise just perform a function on it.

Example:

interface employee {
    first: string,
    last: string
}

function updateEmployees (emp: employee | employee[]) {
    let employees = [];
    if (emp instanceof Array) employees = [emp];
    else employees = emp;
    employees.forEach(function(e){
        return 'something'
    })
}

This seems like it would work to me but is throwing a warning of Type 'employee' is not assignable to type 'any[]'. Property 'length' is missing in type 'employee'.

• javascript
• arrays
• typescript

Share Improve this question Follow edited Jun 4, 2017 at 7:22 Rob♦ 27.4k1616 gold badges8888 silver badges102102 bronze badges asked May 31, 2017 at 0:56 Jon LambJon Lamb 1,47322 gold badges1717 silver badges3030 bronze badges 2

• You switched around the if / else – pushkin Commented May 31, 2017 at 0:59
• This kind of polymorphism is a bad idea in general. Just make updateEmployees take an array. If you want to call it with a single employee, then do so with updateEmployees([emp]). – user663031 Commented Jun 4, 2017 at 7:34

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2 Answers 2

Sorted by: Reset to default 6

Here's the fixed version of your function:

function updateEmployees (emp: employee | employee[]) {
    let employees: employee[] = [];
    if (emp instanceof Array) employees = emp;
    else employees = [emp];
    employees.forEach(function(e){
        return 'something'
    })
}

But it can be shorter:

function updateEmployees(emp: employee | employee[]) {
    (emp instanceof Array ? emp : [emp]).forEach(function(e){
        return 'something'
    })
}

Here is a typesafe function that ensures an array, (using Typescript). It also checks to see if the item is null or undefined, and returns empty array if so, instead of adding it to an array.

function ensureArray<T>(value: T | T[]): T[] {
  if (Array.isArray(value)) {
    return value
  } else if (value === undefined || value === null) {
    return []
  } else {
    return [value]
  }
}

If you DO want an array that contains null or undefined:

function ensureArray<T>(value: T | T[]): T[] {
  if (Array.isArray(value)) {
    return value
  } 
  else {
    return [value]
  }
}

For your example, the usage would be:

function updateEmployees (emp: employee | employee[]) {
    ensureArray(emp).forEach(function(e){
        return 'something'
    })
}

Notice that a null or undefined element will not create a runtime error using the additional check. And the item in your loop is now of type 'employee'.