Prints a formatted string to the standard output. It does not append the newline character at the end. The first argument must a String object. It's same as Go's `fmt.Printf`.
```golang
a := [1, 2, 3]
printf("foo %v", a) // "foo [1, 2, 3]"
```
## sprintf
Returns a formatted string. The first argument must be a String object. It's the same as Go's `fmt.Sprintf`.
Returns the number of elements if the given variable is array, string, map, or module map.
```golang
v := [1, 2, 3]
l := len(v) // l == 3
```
## copy
Creates a copy of the given variable. `copy` function calls `Object.Copy` interface method, which is expected to return a deep-copy of the value it holds.
```golang
v1 := [1, 2, 3]
v2 := v1
v3 := copy(v1)
v1[1] = 0
print(v2[1]) // "0"; 'v1' and 'v2' referencing the same array
print(v3[1]) // "2"; 'v3' not affected by 'v1'
```
## append
Appends object(s) to an array (first argument) and returns a new array object. (Like Go's `append` builtin.) Currently, this function takes array type only.
Tries to convert an object to string object. See [this](https://github.com/d5/tengo/wiki/Variable-Types) for more details on type conversion.
```golang
x := string(123) // v == "123"
```
Optionally it can take the second argument, which will be returned if the first argument cannot be converted to string. Note that the second argument does not have to be string.
```golang
v = string(undefined, "foo") // v == "foo"
v = string(undefined, false) // v == false
```
## int
Tries to convert an object to int object. See [this](https://github.com/d5/tengo/blob/master/docs/runtime-types.md) for more details on type conversion.
```golang
v := int("123") // v == 123
```
Optionally it can take the second argument, which will be returned if the first argument cannot be converted to int. Note that the second argument does not have to be int.
```golang
v = int(undefined, 10) // v == 10
v = int(undefined, false) // v == false
```
## bool
Tries to convert an object to bool object. See [this](https://github.com/d5/tengo/blob/master/docs/runtime-types.md) for more details on type conversion.
```golang
v := bool(1) // v == true
```
## float
Tries to convert an object to float object. See [this](https://github.com/d5/tengo/blob/master/docs/runtime-types.md) for more details on type conversion.
```golang
v := float("19.84") // v == 19.84
```
Optionally it can take the second argument, which will be returned if the first argument cannot be converted to float. Note that the second argument does not have to be float.
```golang
v = float(undefined, 19.84) // v == 19.84
v = float(undefined, false) // v == false
```
## char
Tries to convert an object to char object. See [this](https://github.com/d5/tengo/blob/master/docs/runtime-types.md) for more details on type conversion.
```golang
v := char(89) // v == 'Y'
```
Optionally it can take the second argument, which will be returned if the first argument cannot be converted to float. Note that the second argument does not have to be float.
```golang
v = char(undefined, 'X') // v == 'X'
v = char(undefined, false) // v == false
```
## bytes
Tries to convert an object to bytes object. See [this](https://github.com/d5/tengo/blob/master/docs/runtime-types.md) for more details on type conversion.
```golang
v := bytes("foo") // v == [102 111 111]
```
Optionally it can take the second argument, which will be returned if the first argument cannot be converted to float. Note that the second argument does not have to be float.
```golang
v = bytes(undefined, bytes("foo")) // v == bytes("foo")
v = bytes(undefined, false) // v == false
```
If you pass an int to `bytes()` function, it will create a new byte object with the given size.
```golang
v := bytes(100)
```
## is_string
Returns `true` if the object is string. Or it returns `false`.
## is_int
Returns `true` if the object is int. Or it returns `false`.
## is_bool
Returns `true` if the object is bool. Or it returns `false`.
## is_float
Returns `true` if the object is float. Or it returns `false`.
## is_char
Returns `true` if the object is char. Or it returns `false`.
## is_bytes
Returns `true` if the object is bytes. Or it returns `false`.
## is_error
Returns `true` if the object is error. Or it returns `false`.
## is_undefined
Returns `true` if the object is undefined. Or it returns `false`.
