How to Define Tasks in deno.json for Deno CLI

In deno.json, define tasks inside the tasks object as key-value pairs where keys are task names and values are command strings or arrays. You can then run these tasks with deno task <task-name> to automate common commands.

Syntax

The tasks field in deno.json is an object where each key is a task name and the value is either a string command or an array of strings representing the command and its arguments.

• Task name: The key used to run the task.
• Command string: A shell command to execute.
• Command array: An array where the first element is the command and the rest are arguments.

json

{
  "tasks": {
    "taskName": "command to run",
    "build": ["deno", "run", "--allow-read", "build_script.ts"]
  }
}

Example

This example shows a deno.json file defining two tasks: start to run a server script and test to run tests. You can run them with deno task start or deno task test.

json

{
  "tasks": {
    "start": "deno run --allow-net server.ts",
    "test": ["deno", "test", "--allow-read"]
  }
}

Output

Running task: start
> deno run --allow-net server.ts

Running task: test
> deno test --allow-read

Common Pitfalls

Common mistakes when defining tasks include:

• Using invalid JSON syntax (missing commas or quotes).
• Defining commands as arrays but mixing string and array formats incorrectly.
• Forgetting to run tasks with deno task <task-name>.
• Not granting required permissions in commands, causing runtime errors.

Always validate your deno.json file and test tasks individually.

json

Wrong:
{
  "tasks": {
    "build": "deno run build.ts",  // Missing comma here fixed
    "test": ["deno", "test"]
  }
}

Right:
{
  "tasks": {
    "build": "deno run build.ts",
    "test": ["deno", "test"]
  }
}

Quick Reference

Property	Description	Example
tasks	Object defining named tasks	"tasks": { "start": "deno run app.ts" }
Task name	Key used to run the task	"start"
Command string	Shell command to execute	"deno run --allow-net server.ts"
Command array	Command and arguments as array	["deno", "test", "--allow-read"]
Run task	CLI command to run task	`deno task start`

Key Takeaways

Define tasks inside the "tasks" object in deno.json as command strings or arrays.

Run tasks using the CLI command: deno task .

Ensure JSON syntax is correct to avoid errors.

Use arrays for commands with multiple arguments for clarity.

Grant necessary permissions in commands to prevent runtime failures.