The interpreter pattern is a behavioral design pattern used to define language grammar and to establish an interpreter to process sentences in the language. It realizes the parsing and execution of expressions by building an abstract syntax tree (AST) and recursively interpreting each node. It is commonly found in template engines, DSL parsers, and formula calculations. Its structure includes abstract expressions, terminator expressions, nonterminator expressions, and context. Applicable scenarios include custom query language parsing, small script engine, expression evaluation, and configuration file parsing. When implementing it, you need to pay attention to controlling complexity, handling performance issues, improving error handling mechanisms, and avoiding repeated creation of wheels.

JavaScript's Interpreter Pattern is often misunderstood as an obscure design pattern. In fact, it is very common in actual development, especially when you need to parse custom languages or expressions. If you have written a simple template engine, a DSL (domain-specific language) parser, or done formula calculation logic, you may have used this pattern.

Let’s talk about how to understand and use the interpreter mode in JavaScript from several perspectives.

What is interpreter mode?

The interpreter pattern is a behavioral design pattern, mainly used to define the grammar of a language and to establish an interpreter to process the sentences of this grammar. To put it bluntly, it means turning some "rules" into objects that can be understood and executed by programs.

For example, you want to implement a simple mathematical expression calculator:

 "3 4 * 2"

You cannot directly let JS eval the string (although it can be implemented but not safe), you need to parse the expression yourself, split it into numbers and operators, and then calculate it according to priority. At this time you need an "interpreter" to do the job.

The core idea is: build an abstract syntax tree (AST), and then "explain" the meaning of each node in a recursive way.

Basic structure of interpreter mode

The interpreter pattern usually contains the following parts:

• Abstract expression : Define a common interface, usually interpret() method.
• Terminal Expression : represents the smallest unit in syntax, such as variables, numbers, etc.
• Non-terminal expression : used to combine other expressions, such as addition and multiplication.
• Context : Provides global information, such as variable values, environment configuration, etc.

For example, if you want to explain an expression similar to "xy * 3" :

• x and y are terminator expressions;
• and * are non-terminal expressions;
• The context saves variable values such as x=5, y=2 .

In what scenario will the interpreter mode be used?

1. Custom query language parsing For example, when you are working on a data filtering system, the user enters a statement similar to "age > 20 AND name LIKE 'J%'" . You can use the interpreter mode to build the corresponding query logic.

2. Small script engine If you are developing a visual programming tool that allows users to enter simple scripts to control the process, it can also be implemented with an interpreter.

3. Expression evaluation For example, the coupon rules in the e-commerce system: "20 off when you spend 100 or get B for free", or "buy A and get B for free", these can be modeled using an interpreter and dynamically determine whether the conditions are met.

4. Configuration file parsing Some systems use custom-format configuration files, such as .dsl files, and the interpreter can help you translate these configurations into logic that the program can execute.

Issues that need to be paid attention to when implementing

• Complexity control If your language is too complex, such as supporting nesting, function calls, variable scope, etc., the interpreter pattern may become difficult to maintain. This is more suitable to use mature parsing libraries (such as ANTLR) or simply translate them into JavaScript expressions.

• Performance issues Every time you have to build AST and then traverse and execute, the efficiency is definitely not as efficient as native code. If the performance requirements are high, you can consider cache interpretation results or compile optimization.

• Error handling should be meticulous The expressions entered by the user may have syntax errors, variables are not defined, etc., so an exception capture and prompt mechanism must be done well.

• Don't make wheels repeatedly Many times there are tools that can be replaced, such as:

  • Use Function constructor (but be careful)
  • Handle math expressions with math.js
  • Use regular expressions to make simple matches

Basically that's it. Interpreter mode is not something you have to master, but it is very useful in certain scenarios. The key is how to map the structure of "language" into object models and interpret them in a unified way. Once you master this idea, you will find that many seemingly complex logic can actually be organized in a very clear way.

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