# String

A fixed-length sequence of UTF-16 code units.

The String API works very much like JavaScript's (MDN), with the notable difference that the string type is an actual alias of String.

# Static members

  • function fromCharCode(unit: i32, surr?: i32): string
    

    Creates a one character long string from the specified UTF-16 code units.

  • function fromCharCodes(units: u16[]): string
    

    Creates a string from a sequence of UTF-16 code units.

  • function fromCodePoint(code: i32): string
    

    Creates a one character long string from the specified UTF-8 code point.

  • function fromCodePoints(codes: i32[]): string
    

    Creates a string from a sequence of UTF-8 code points.

# Instance members

# Fields

  • readonly length: i32
    
    The length of the string in UTF-16 code units.

# Methods

  • function charAt(pos: i32): string
    

    Gets the UTF-16 code unit at the specified position as a single character string. Returns "" (empty string) if out of bounds.

  • function charCodeAt(pos: i32): i32
    

    Gets the UTF-16 code unit at the specified position as a number. Returns -1 if out of bounds.

  • function codePointAt(pos: i32): i32
    

    Gets the UTF-8 code point at the specified (UTF-16 code unit) position as a number, possibly combining multiple successive UTF-16 code units. Returns -1 if out of bounds.

  • function concat(other: string): string
    

    Concatenates this string with another string, in this order, and returns the resulting string.

  • function endsWith(search: string, end?: i32): bool
    

    Tests if the strings ends with the specified string. If specified, end indicates the position at which to stop searching, acting as if it is the length of the string.

  • function includes(search: string, start?: i32): bool
    

    Tests if the string includes the search string. If specified, start indicates the position at which to begin searching.

  • function indexOf(search: string, start?: i32): i32
    

    Gets the first index of the specified search string within the string, or -1 if not found. If specified, start indicates the position at which to begin searching.

  • function lastIndexOf(search: string, start?: i32): i32
    

    Gets the last index of the specified search string within the string, or -1 if not found. If specified, pos indicates the position at which to begin searching from right to left.

  • function padStart(length: i32, pad: string): string
    

    Pads the string with the contents of another string, possibly multiple times, until the resulting string reaches the specified length, returning the resulting string.

  • function padEnd(length: i32, pad: string): string
    

    Pads the string with the contents of another string, possibly multiple times, until the resulting string reaches the specified length, returning the resulting string.

  • function repeat(count?: i32): string
    

    Repeats the string count times and returns the concatenated result.

  • function replace(search: string, replacement: string): string
    

    Replaces the first occurrence of search with replacement.

  • function replaceAll(search: string, replacement: string): string
    

    Replaces all occurrences of search with replacement.

  • function slice(start: i32, end?: i32): string
    

    Returns the region of the string from start inclusive to end exclusive, as a new string. If omitted, end defaults to the end of the string.

  • function split(separator?: string, limit?: i32): string[]
    

    Splits the string at each occurrence of separator and returns the result as a new array of strings that has a maximum of limit values. If limit is omitted, no limit is assumed. If separator is omitted or not present in the string, the string becomes the sole element of the array. If separator is an empty string, the split is performed between code units (not code points), potentially destroying surrogate pairs.

  • function startsWith(search: string, start?: i32): bool
    

    Tests if the string starts with the specified string. If specified, pos indicates the position at which to begin searching, acting as the start of the string.

  • function substring(start: i32, end?: i32): string
    

    Gets the part of the string in between start inclusive and end exclusive as a new string.

  • function toString(): this
    

    Returns the string.

  • function trim(): string
    

    Removes white space characters from both the start and the end of the string, returning the resulting string.

  • function trimStart(): string
    function trimLeft(): string
    

    Removes white space characters from the start of the string, returning the resulting string.

  • function trimEnd(): string
    function trimRight(): string
    

    Removes white space characters from the end of the string, returning the resulting string.

# Encoding API

# UTF8

When integrating with an environment that uses UTF-8, the following helpers can be used to quickly re-encode String data.

  • function String.UTF8.byteLength(str: string, nullTerminated?: bool): i32
    

    Calculates the byte length of the specified string when encoded as UTF-8, optionally null terminated.

  • function String.UTF8.encode(str: string, nullTerminated?: bool): ArrayBuffer
    

    Encodes the specified string to UTF-8 bytes, optionally null terminated.

  • function String.UTF8.encodeUnsafe(str: usize, len: i32, buf: usize, nullTerminated?: bool): usize
    

    Encodes the specified raw string to UTF-8 bytes, opionally null terminated. Returns the number of bytes written.

  • function String.UTF8.decode(buf: ArrayBuffer, nullTerminated?: bool): string
    

    Decodes the specified buffer from UTF-8 bytes to a string, optionally null terminated.

  • function String.UTF8.decodeUnsafe(
      buf: usize,
      len: usize,
      nullTerminated?: bool
    ): string
    

    Decodes raw UTF-8 bytes to a string, optionally null terminated.

TIP

Note that any ArrayBuffer return value is a pointer to the buffer's data internally and thus can be passed to let's say a C-function directly. However, if the pointer is meant to live longer than the immediate external function call, the lifetime of the buffer must be tracked so it doesn't become collected prematurely with the data becoming invalid.

# UTF16

The following mostly exist to have a safe way to copy between Strings and ArrayBuffers, but doesn't involve a re-encoding step.

  • function String.UTF16.byteLength(str: string): i32
    

    Calculates the byte length of the specified string when encoded as UTF-16.

  • function String.UTF16.encode(str: string): ArrayBuffer
    

    Encodes the specified string to UTF-16 bytes.

  • function String.UTF16.encodeUnsafe(str: usize, len: i32, buf: usize): usize
    

    Encodes the specified raw string to UTF-16 bytes. Returns the number of bytes written.

  • function String.UTF16.decode(buf: ArrayBuffer): string
    

    Decodes the specified buffer from UTF-16 bytes to a string.

  • function String.UTF16.decodeUnsafe(buf: usize, len: usize): string
    

    Decodes raw UTF-16 bytes to a string.

# Considerations

AssemblyScript stores strings the same way JavaScript does, i.e. by encoding them using UTF-16. In UTF-16, certain Unicode code points are represented by two UTF-16 code units called surrogate pairs. However, just like in JavaScript, strings are not sanitized so lone surrogates can appear in a string, which are technically ill-formed UTF-16. This is done to mimic JavaScript as closely as possible, and avoids string re-encoding when calling JavaScript APIs from WebAssembly. Whether future WebAssembly specifications will help in this regard is still subject to discussion.