nxtpression

Friendly observable-based template expression library.

const { of } = require('rxjs')
const nxtpr = require('nxtpression')

nxtpr.produceObservable('{{names|greet}}', {
  names: of("Mike", "Ike"),
  greet: x => `Hi ${x}.`
}).subscribe(console.log)
// Hi Mike.
// Hi Ike.

You can get much fancier than that though!

🚼 Syntax Reference

NOTE: Every expression represents a stream of values. Mix and match as you see wish!

{{ [{ [currentKey]: currentInput }] | map(currentlySelectedTransform | processResult(currentScheduler)) }}

Array [foo, bar]

{{ [1] }}
{{ [1, 2] }}
{{ [foo, 2] }}

Boolean true / false

{{ true }}
{{ false }}

Function Call foo(bar)

Note that no currying is performed.

{{ of(1, 2) }}
{{ seq(period, num) }}
{{ mul(2)(3) }}

Index foo[bar]

{{ a[b] }}

Member foo.bar

{{ foo.bar }}

Null null

{{ null }}

Number 1.23

WARNING: this does not support scientific notation like 1e3

{{ 12 }}
{{ 1.2345 }}

Object { foo: foo, [bar]: baz }

Supports both static and dynamic keys

WARNING: does not yet support the same-name utility syntax { foo }

{{ {} }}
{{ { a: 1 } }}
{{ { a: foo } }}
{{ { a: 1, b: 2 } }}
{{ { [a]: 1 } }}
{{ { [a]: 1, [b]: 2 } }}
{{ { a: 1, [b]: 2 } }}

Pipe foo | bar

{{ 2 | mul(3) }}
{{ "ff" | parseHex }}
{{ stream | sub(5) | mul(2) | add(1) }}
{{ x | map(mul(2) | add(1)) }}

Reference foo

{{ a }}
{{ myVar }}

String "foo {{ bar }}"

A bit more complicated, since nested templates are supported. Strings can be delimited by either 's or "s.

WARNING: templates inside non-empty strings will have to convert all emissions to String.

WARNING: can't use ` delimiters

{{ "" }}  // ''
{{ '' }}  // ''
{{ "a" }} // 'a'
{{ "{{ "" }}" }}  // ''
{{ "{{ 1 }}" }}   // 1
{{ "a{{ 1 }}" }}  // 'a1'
{{ "{{ "a" }}" }} // 'a'
{{ "hell{{ "o" }} world" }} // 'hello world'

Undefined undefined

{{ undefined }}

🕯️ Core API

The core design is really simple.

1. Tokenize (look at source code and find where the tokens are - information is found)
2. Parse (take the tokens and produce a tree structure of nodes - relationships appear)
3. Compile (create a factory function, mapping a context to an observable - code becomes callable)
4. Inject a context (inject streams and values to produce a composite stream as defined in the template)
5. Subscribe the stream! 🎳

For more information on steps 1-3, watch the destroyallsoftware screencast linked in the bottom for a terrific 30 minute short introduction on writing a compiler from scratch. For more information on steps 4-5, learn RxJS.

tokenize: (source) => tokens (throws on unexpected token)

Tokenizes the source string and returns an Array of tokens.

Notably, a string is not a string until it has been parsed. An actual string is formed during parsing.

• type: String; For a list of possible values, see tokenize.js
• start: Number; The index in the source string where the token starts.
• body: String; The full slice of text as copied from the source string.

parseFromTokens: (source, tokens) => AST (throws on invalid semantics)

Parses tokens into an abstract syntax tree - a recursive structure of nodes.

A node has the following shape: { type, ...properties }

• func - a function (path: optional parent node, args - array of nodes called with)
• ref - a reference (name: token.body, col: token.start)
• boolean - a boolean (value: the parsed Boolean value)
• object - an object (props: array of { path, expr } nodes - there are two types of path, see below)
  • constprop - a statically named property (value: the static name of the property)
  • dynprop - a dynamically named property (expr: a node representing an expression for the property name)
• stringparts - a string possibly containing a nxtpression (parts: a list of nodes of varying type)
  • string - a string literal (body: the raw string value)
• index - an index into something (node: the node representing something to index a property off of, expr: the node representing the value to index off of the thing)
• member - a member access (node: the node representing something to access a property off of, property: the name of the property to access)
• array - an array (items: array of nodes reprenting the values in the array)
• number - a number (value: the parsed Number value)
• pipe - a function piping the left hand side to the right hand side (parts: the nodes representing the objects to pipe through. Most of the time, the first node will reference a value from context and the other nodes are functions to pipe that value through)
• null - the value null (n/a)
• undefined - the value undefined (n/a)

compileFromAST: (source, AST, options) => (context => Observable) (throws on undefined variable access if enabled)

Compiles a template string from an AST.

The options object supports the following properties:

• throwOnUndefinedVariableAccess - if true, will throw when a value referenced in the context is undefined (default: false)

IGNORE: Symbol

Consider running {{ 4 | ignoreEven | doSideEffect }}, with ignoreEven: x => x % 2 === 0 ? IGNORE : x. This will not perform the side effect, because once an IGNORE symbol is discovered, the symbol will be emitted immediately.

WARNING: this WILL still emit once - with the IGNORE symbol - in order to ensure that observables complete.

♿ 🕹️ Utility API

There are some nice utilities to make your life easier. Basically just some wrappers around the core API and a heuristic to help determining whether a string might contain a template.

isNotATemplate: (string) => Boolean (does not throw)

If string is not a String containing '{{', this will return true.

WARNING: this is only a heuristic when returning false. If you need to be certain, please parse the string and make sure no error is thrown.

parseFromSource: (source) => AST (throws on unexpected token or invalid semantics)

This utility function will tokenize and parseFromTokens in one go.

compileTemplate: (source, options) => (context => Observable) (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

This utility function will tokenize, parseFromTokens and compileFromAST in one go. For a list of options, see compileFromAST.

produceObservable: (source, context, options) => Observable (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

This utility function will just compileTemplate(source, options)(context). For a list of options, see compileFromAST.

resolveTemplate: (source, context, options) => Promise (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

This creates a Promise awaiting the first value emitted by a template compiled from source directly when applied with the given context. For a list of options, see compileFromAST.

🍇 Object Templates API

An object template is a recursive structure of arrays and objects containing template strings, e.g.

{
  a: [{ b: '{{ 1 }}' }],
  c: { d: '{{ 2 }}' },
  e: '{{ 3 }}'
} // => { a: [{ b: 1 }], c: { d: 2 }, e: 3 }

However, it does not work recursively in returned values

resolveObjectTemplate({ a: '{{ t }}' }, { t: '{{ 1 }}' }) // { a: '{{ 1 }}' }

compileObjectTemplate: (obj, options) => (context => Observable) (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

Compiles and combines all templates in an object or array recursively. For a list of options, see compileFromAST.

produceObjectObservable: (obj, context, options) => Observable (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

This utility function will just compileObjectTemplate(source, options)(context). For a list of options, see compileFromAST.

resolveObjectTemplate: (obj, context, options) => Promise (throws on unexpected token or invalid semantics or - if enabled, undefined variable access)

Like resolveTemplate but for object templates, this Promises the latest value of all template strings once they all have emitted at least once. For a list of options, see compileFromAST.

🔌 Installation

With npm installed;

npm i nxtpression

🏆 Credit

Inspired by destroyallsoftware’s a compiler from scratch.

Prior art includes jinja2 and nxtedition templates.