# jq: JSON Query and Extraction Reference

**Goal: Extract specific data from JSON without reading entire file.**

## The Essential Pattern

```bash
jq '.field' file.json
```

Use `-r` flag for raw output (removes quotes from strings):
```bash
jq -r '.field' file.json
```

**Use `-r` by default for string values** - cleaner output.

---

# Core Patterns (80% of Use Cases)

## 1. Extract Top-Level Field
```bash
jq -r '.version' package.json
jq -r '.name' package.json
```

## 2. Extract Nested Field
```bash
jq -r '.dependencies.react' package.json
jq -r '.scripts.build' package.json
jq -r '.config.database.host' config.json
```

## 3. Extract Multiple Fields
```bash
jq '{name, version, description}' package.json
```
Creates object with just those fields.

Or as separate lines:
```bash
jq -r '.name, .version' package.json
```

## 4. Extract from Array by Index
```bash
jq '.[0]' array.json           # First element
jq '.items[2]' data.json        # Third element
```

## 5. Extract All Array Elements
```bash
jq '.[]' array.json             # All elements
jq '.items[]' data.json         # All items
```

## 6. Extract Field from Each Array Element
```bash
jq -r '.dependencies | keys[]' package.json      # All dependency names
jq -r '.items[].name' data.json                  # Name from each item
```

## 7. Filter Array by Condition
```bash
jq '.items[] | select(.active == true)' data.json
jq '.items[] | select(.price > 100)' data.json
```

## 8. Get Object Keys
```bash
jq -r 'keys[]' object.json
jq -r '.dependencies | keys[]' package.json
```

## 9. Check if Field Exists
```bash
jq 'has("field")' file.json
```

## 10. Handle Missing Fields (Use // for Default)
```bash
jq -r '.field // "default"' file.json
```

---

# Common Real-World Workflows

## "What version is this package?"
```bash
jq -r '.version' package.json
```

## "What's the main entry point?"
```bash
jq -r '.main' package.json
```

## "List all dependencies"
```bash
jq -r '.dependencies | keys[]' package.json
```

## "What version of React?"
```bash
jq -r '.dependencies.react' package.json
```

## "List all scripts"
```bash
jq -r '.scripts | keys[]' package.json
```

## "Get specific script command"
```bash
jq -r '.scripts.build' package.json
```

## "Check TypeScript compiler options"
```bash
jq '.compilerOptions' tsconfig.json
```

## "Get target from tsconfig"
```bash
jq -r '.compilerOptions.target' tsconfig.json
```

## "List all services from docker-compose JSON"
```bash
jq -r '.services | keys[]' docker-compose.json
```

## "Get environment variables for a service"
```bash
jq '.services.api.environment' docker-compose.json
```

---

# Advanced Patterns (20% Use Cases)

## Combine Multiple Queries
```bash
jq '{version, deps: (.dependencies | keys)}' package.json
```

## Map Array Elements
```bash
jq '[.items[] | .name]' data.json          # Array of names
```

## Count Array Length
```bash
jq '.items | length' data.json
jq '.dependencies | length' package.json
```

## Sort Array
```bash
jq '.items | sort_by(.name)' data.json
```

## Group and Transform
```bash
jq 'group_by(.category)' data.json
```

## Complex Filter
```bash
jq '.items[] | select(.active and .price > 100) | .name' data.json
```

---

# Pipe Composition

jq uses `|` for piping within queries:
```bash
jq '.items | map(.name) | sort' data.json
```

Can also pipe to shell commands:
```bash
jq -r '.dependencies | keys[]' package.json | wc -l     # Count dependencies
jq -r '.dependencies | keys[]' package.json | sort      # Sorted dependency list
```

---

# Common Flags

- `-r` - Raw output (no quotes) - **USE THIS FOR STRINGS**
- `-c` - Compact output (single line)
- `-e` - Exit with error if output is false/null
- `-S` - Sort object keys
- `-M` - Monochrome (no colors)

**Default to `-r` for string extraction.**

---

# Handling Edge Cases

## If Field Might Not Exist
```bash
jq -r '.field // "not found"' file.json
```

## If Result Might Be Null
```bash
jq -r '.field // empty' file.json          # Output nothing if null
```

## If Array Might Be Empty
```bash
jq -r '.items[]? // empty' file.json       # ? suppresses errors
```

## Multiple Possible Paths
```bash
jq -r '.field1 // .field2 // "default"' file.json
```

---

# Error Handling

If field doesn't exist:
```bash
# BAD: jq '.nonexistent' file.json
# → null (but no error)

# GOOD: Check existence first
jq -e 'has("field")' file.json && jq '.field' file.json
```

Or use default:
```bash
jq -r '.field // "not found"' file.json
```

---

# Integration with Other Tools

## With ast-grep
```bash
# Get dependencies, then search code for usage
jq -r '.dependencies | keys[]' package.json | while read dep; do
  rg -l "from ['\"]$dep['\"]"
done
```

## With Edit Tool
Common workflow:
1. Use jq to extract current value
2. Modify value
3. Use Edit tool to update JSON (or jq for complex updates)

## Reading STDIN
```bash
echo '{"key":"value"}' | jq -r '.key'
```

---

# Best Practices

## 1. Always Use -r for String Fields
```bash
# BAD:  jq '.version' package.json  → "1.0.0" (with quotes)
# GOOD: jq -r '.version' package.json  → 1.0.0 (raw)
```

## 2. Test Queries on Small Examples First
```bash
echo '{"test":"value"}' | jq -r '.test'
```

## 3. Use // for Defaults
```bash
jq -r '.field // "default"' file.json
```

## 4. Use keys[] for Object Properties
```bash
jq -r 'keys[]' object.json
```

## 5. Combine with Shell Pipes
```bash
jq -r '.dependencies | keys[]' package.json | grep react
```

---

# Quick Reference

## Most Common Commands

```bash
# Single field
jq -r '.field' file.json

# Nested field
jq -r '.parent.child' file.json

# Array element
jq '.array[0]' file.json

# All array elements
jq '.array[]' file.json

# Object keys
jq -r 'keys[]' file.json

# Filter array
jq '.array[] | select(.field == "value")' file.json

# Multiple fields
jq '{field1, field2}' file.json

# With default
jq -r '.field // "default"' file.json
```

---

# When to Use Read Instead

Use Read tool when:
- File is < 50 lines
- Need to see overall structure
- Making edits (need full context)
- Exploring unknown JSON structure

Use jq when:
- File is large
- Know exactly what field(s) are needed
- Want to save context tokens

---

# Summary

**Default pattern:**
```bash
jq -r '.field' file.json
```

**Key principles:**
1. Use `-r` for string output (raw, no quotes)
2. Use `.` notation for nested fields
3. Use `[]` for array access
4. Use `//` for defaults
5. Use `keys[]` for object properties
6. Pipe with `|` inside jq, pipe to shell after

**Massive context savings: Extract only what is needed instead of reading entire JSON files.**