---
title: Runtime Types
slug: reference/runtime-types
docTags: 
createdAt: 2025-09-23T15:06:05.439Z
---

## Tengo Runtime Types

- **Int**: signed 64bit integer
- **String**: string
- **Float**: 64bit floating point
- **Bool**: boolean
- **Char**: character (`rune` in Go)
- **Bytes**: byte array (`[]byte` in Go)
- **Array**: objects array (`[]Object` in Go)
- **ImmutableArray**: immutable object array (`[]Object` in Go)
- **Map**: objects map with string keys (`map[string]Object` in Go)
- **ImmutableMap**: immutable object map with string keys (`map[string]Object` in Go)
- **Time**: time (`time.Time` in Go)
- **Error**: an error with underlying Object value of any type
- **Undefined**: undefined

### Type Conversion/Coercion Table

| src\dst   | Int       | String           | Float      | Bool       | Char    | Bytes      | Array | Map   | Time          | Error | Undefined |
| --------- | --------- | ---------------- | ---------- | ---------- | ------- | ---------- | ----- | ----- | ------------- | ----- | --------- |
| Int       | -         | *strconv*        | float64(v) | !IsFalsy() | rune(v) | **X**      | **X** | **X** | *time.Unix()* | **X** | **X**     |
| String    | *strconv* | -                | *strconv*  | !IsFalsy() | **X**   | \[]byte(s) | **X** | **X** | **X**         | **X** | **X**     |
| Float     | int64(f)  | *strconv*        | -          | !IsFalsy() | **X**   | **X**      | **X** | **X** | **X**         | **X** | **X**     |
| Bool      | 1 / 0     | "true" / "false" | **X**      | -          | **X**   | **X**      | **X** | **X** | **X**         | **X** | **X**     |
| Char      | int64(c)  | string(c)        | **X**      | !IsFalsy() | -       | **X**      | **X** | **X** | **X**         | **X** | **X**     |
| Bytes     | **X**     | string(y)        | **X**      | !IsFalsy() | **X**   | -          | **X** | **X** | **X**         | **X** | **X**     |
| Array     | **X**     | "\[...]"         | **X**      | !IsFalsy() | **X**   | **X**      | -     | **X** | **X**         | **X** | **X**     |
| Map       | **X**     | ""               | **X**      | !IsFalsy() | **X**   | **X**      | **X** | -     | **X**         | **X** | **X**     |
| Time      | **X**     | String()         | **X**      | !IsFalsy() | **X**   | **X**      | **X** | **X** | -             | **X** | **X**     |
| Error     | **X**     | "error: ..."     | **X**      | false      | **X**   | **X**      | **X** | **X** | **X**         | -     | **X**     |
| Undefined | **X**     | **X**            | **X**      | false      | **X**   | **X**      | **X** | **X** | **X**         | **X** | -         |

*\* **X**: No conversion; Typed value functions for *`Variable`* will return zero values.*
*\* strconv: converted using Go's conversion functions from *`strconv`* package.*
*\* IsFalsy(): use *[Object.IsFalsy()](docId\:smONYFzO12nj6JXYY8eu4)* function*
*\* String(): use *`Object.String()`* function* *\* time.Unix(): use *`time.Unix(v, 0)`* to convert to Time*

### Object.IsFalsy()

`Object.IsFalsy()` interface method is used to determine if a given value should evaluate to `false` (e.g. for condition expression of `if` statement).

- **Int**: `n == 0`
- **String**: `len(s) == 0`
- **Float**: `isNaN(f)`
- **Bool**: `!b`
- **Char**: `c == 0`
- **Bytes**: `len(bytes) == 0`
- **Array**: `len(arr) == 0`
- **Map**: `len(map) == 0`
- **Time**: `Time.IsZero()`
- **Error**: `true` *(Error is always falsy)*
- **Undefined**: `true` *(Undefined is always falsy)*

### Type Conversion Builtin Functions

- `string(x)`: tries to convert `x` into string; returns `undefined` if failed
- `int(x)`: tries to convert `x` into int; returns `undefined` if failed
- `bool(x)`: tries to convert `x` into bool; returns `undefined` if failed
- `float(x)`: tries to convert `x` into float; returns `undefined` if failed
- `char(x)`: tries to convert `x` into char; returns `undefined` if failed
- `bytes(x)`: tries to convert `x` into bytes; returns `undefined` if failed
  - `bytes(N)`: as a special case this will create a Bytes variable with the given size `N` (only if `N` is int)
- `time(x)`: tries to convert `x` into time; returns `undefined` if failed
- See [Builtins](docId\:UUfOzxXgUZPCBSx61qIQb) for the full list of builtin functions.

### Type Checking Builtin Functions

- `is_string(x)`: returns `true` if `x` is string; `false` otherwise
- `is_int(x)`: returns `true` if `x` is int; `false` otherwise
- `is_bool(x)`: returns `true` if `x` is bool; `false` otherwise
- `is_float(x)`: returns `true` if `x` is float; `false` otherwise
- `is_char(x)`: returns `true` if `x` is char; `false` otherwise
- `is_bytes(x)`: returns `true` if `x` is bytes; `false` otherwise
- `is_array(x)`: return `true` if `x` is array; `false` otherwise
- `is_immutable_array(x)`: return `true` if `x` is immutable array; `false` otherwise
- `is_map(x)`: return `true` if `x` is map; `false` otherwise
- `is_immutable_map(x)`: return `true` if `x` is immutable map; `false` otherwise
- `is_time(x)`: return `true` if `x` is time; `false` otherwise
- `is_error(x)`: returns `true` if `x` is error; `false` otherwise
- `is_undefined(x)`: returns `true` if `x` is undefined; `false` otherwise
- See [Builtins](docId\:UUfOzxXgUZPCBSx61qIQb) for the full list of builtin functions.